RISK FACTORS

         An investment in our common stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below before making a decision to invest in our common stock. Our business, operating results, financial condition or prospects could be materially and adversely affected by any of these risks and uncertainties. In that case, the trading price of our common stock could decline and you might lose all or part of your investment. In addition, the risks and uncertainties discussed below are not the only ones we face. Our business, operating results, financial condition or prospects could also be harmed by risks and uncertainties not currently known to us or that we currently do not believe are material, and these risks and uncertainties could result in a complete loss of your investment. In assessing the risks and uncertainties described below, you should also refer to the other information contained in this prospectus (as supplemented or amended), including our consolidated financial statements and the related notes thereto, before making a decision to invest in our common stock.

Risks Related to our Business

Our recent growth may not be indicative of our future growth and, if we continue to grow, we may not be able to manage our growth effectively.

        We have recently experienced a period of rapid growth in our headcount and operations. In particular, we grew from 58 employees as of December 31, 2011, to 408 fulltime employees as of September 30, 2017, and we have also significantly increased the number of emails processed by our platform over the last several years. We anticipate that we will continue to significantly expand our operations and headcount in the near term. Our growth has placed, and future growth will place, a significant strain on our management, technical, administrative, operational and financial infrastructure. Our success will depend in part on our ability to manage this growth effectively. To manage the expected growth of our operations and personnel, we will need to continue to improve our management, technical, administrative, operational and financial controls and our reporting systems and procedures. Failure to effectively manage our growth could result in difficulty or delays in effectively scaling our platform to handle increased email volumes, declines in quality or customer satisfaction, increases in costs, difficulties in introducing new features or other operational difficulties. Any of these difficulties could adversely impact our business and results of operations.

If we are unable to sustain our revenue growth rate, we may not achieve or maintain profitability in the future.

        We have experienced rapid revenue growth over recent years, with revenue of $42.8 million, $58.5 million and $79.9 million in 2014, 2015 and 2016, respectively. Although we have experienced rapid revenue growth historically, we may not continue to grow as rapidly in the future and our revenue growth rates may decline. Any success that we may experience in the future will depend in large part on our ability to, among other things:

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maintain and expand our customer base;



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increase revenue from existing customers through increased or broader use of our platform within their organizations;



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improve the performance and capabilities of our platform through research and development;



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continue to successfully expand our business domestically and internationally; and



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successfully compete with other companies.

        If we are unable to maintain consistent revenue or revenue growth, our stock price could be volatile or decline, and we may not achieve or maintain profitability. You should not rely on our revenue for any prior quarterly or annual periods as any indication of our future revenue or revenue growth.

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If we are unable to attract new customers, retain existing customers or increase sales both to new and existing customers, our business and results of operations will be affected adversely.

        To succeed, we must continue to attract and retain customers and increase sales to new and existing customers. We price our services on a tiered subscription model based on the customer's use of our services (principally, email volumes), and therefore the revenue we generate from our customers depends on their use of our services. We have only a limited salesforce and to date have relied primarily on our self-service model to generate our revenue. The amount that new and existing customers purchase, renew and use our services depends on a number of factors, including those outside of our control.

        We may fail to attract new customers, retain existing customers or increase sales to new or existing customers as a result of a number of factors, including: reductions in our current or potential customers' spending levels; competitive factors affecting the cloud-based business software market, including the introduction of competing platforms, discount pricing and other strategies that may be implemented by our competitors; our ability to execute on our growth strategy and operating plans; a decline in our customers' level of satisfaction with our platform and customers' usage of our platform or an increased perception by our customers' that they can manage their email distribution and marketing efforts internally or otherwise without use of our platform; reductions in the use of email as a digital communication channel; the difficulty and cost to switch to a competitor may not be significant for many of our customers; changes in our relationships with third parties, including our strategic partners and payment processors; the timeliness and success of new services and functionality we may offer now or in the future, including our Marketing Campaigns service that we introduced in late 2015; concerns relating to actual or perceived security breaches; the frequency and severity of any system outages; and technological changes or problems. In addition, we believe a relatively small number of businesses have more than one unique paying account with us, and we count each of these accounts as a separate customer. We believe our number of customers is an important measure for evaluating our business. Because some businesses have more than one unique paying account with us and we count each of these accounts as a separate customer, the number of our customers set forth in this prospectus for any period is not necessarily indicative of the number of unique businesses from which we received revenue during any such period. We rely on our reputation and recommendations from key customers in order to promote our platform. The loss of any of our key customers could have a significant impact on our business reputation and our ability to obtain new customers. In addition, acquisitions of our customers could lead to cancellation of our contracts with those customers or by the acquiring companies.

        Failure to attract new customers, retain existing customers or increase sales to customers will harm our business and results of operations.

Our limited operating history in new and developing markets and our rapid growth make it difficult to evaluate our current business and future prospects.

        We introduced our Email API service in 2009, our Marketing Campaigns service in late 2015 and our Expert Services in 2016. The majority of our revenue growth has occurred in the last few years and was derived from the sale of subscriptions to our Email API service. This short operating history and our rapid growth make it difficult to evaluate our future prospects. Our ability to forecast our future operating results is subject to a number of uncertainties, including our ability to plan for and model future growth, particularly with respect to our most recently introduced services. If our assumptions regarding these uncertainties are incorrect or change in reaction to changes in our markets, or if we do not address these risks successfully, our operating and financial results could differ materially from our historical and expected operating and financial results, our business could suffer and the trading price of our stock may decline.

        We also operate in new and developing markets that may not continue to develop as we expect. You should consider our future prospects in light of the challenges and uncertainties that we face, including the

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fact that our business has grown rapidly and it may not be possible to discern fully the trends that we are subject to, that we operate in new and developing markets and that elements of our business strategy are new and subject to ongoing development. We have encountered and will continue to encounter risks and difficulties frequently experienced by growing companies in rapidly changing industries, including increasing and unforeseen expenses as we continue to grow our business. If we do not manage these risks successfully, our business and results of operations will be harmed.

If we are unable to increase adoption of our platform through our self-service model, our business, results of operations and financial condition may be adversely affected.

        Historically, we have relied on the adoption of our platform through our self-service model for a significant majority of our revenue. We have only a limited salesforce. Although we believe our business model can continue to scale without a significantly larger salesforce, our self-service model may not continue to be as effective as we anticipate, and the absence of a large direct sales function may impede our future growth.

        As we continue to scale our business, we may choose to invest in a larger direct salesforce to reach additional customers and grow our revenue. Our ability to manage a larger direct salesforce is uncertain. Identifying and recruiting additional qualified sales personnel and managing and training them once hired would require significant time, expense and attention and would significantly impact our business model. In addition, adding additional salesforce would considerably change our cost structure and results of operations, and we may have to reduce other expenses, such as our research and development expense, in order to accommodate a corresponding increase in marketing and sales expense and attain and maintain profitability. If our lack of a large direct salesforce limits us from reaching additional customers and growing our revenue and we are unable to hire, develop and retain talented sales personnel in the future, our revenue growth and results of operations may be harmed.

Our growth depends in part on the success of our strategic relationships with third parties to sell our services.

        We have established strategic relationships with a number of other companies, including public cloud infrastructure providers and ecommerce platforms, software vendors that offer complementary products and with which we co-sell our services and digital marketing agencies that resell our services to their clients. In order to grow our business, we anticipate that we will continue to establish and maintain these strategic relationships. Identifying strategic partners, and negotiating and documenting relationships with them, requires significant time and resources. Our competitors may be effective in providing incentives to third parties to favor their products or services over ours or to prevent or reduce subscriptions to our platform. In addition, acquisitions of our strategic partners by our competitors, or acquisition of our competitors by our strategic partners, could result in a decrease in the number of our current and potential customers, as our strategic partners may no longer facilitate the adoption of our platform by potential customers.

        If we are unsuccessful in establishing or maintaining our strategic relationships with third parties, our ability to compete or to grow our revenues could be impaired and our operating results could suffer. Even if we are successful in our strategic relationships, we cannot assure you that these relationships will result in increased usage of our services or increased revenues.

Our future success depends in part on our ability to continue to drive adoption of our platform and services by international customers, and our international operations and sales to customers with international operations expose us to risks inherent in international sales.

        We generated more than 36% of our revenue in each of the last three years from customers located outside the United States. The future success of our business will depend, in part, on our ability to continue to expand our customer base worldwide and to sell additional services to international customers.

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If we are unable to successfully market our platform to or localize our services for international customers, then our business, results of operations and financial condition may be adversely affected and our growth may be constrained.

        We have limited experience operating in international markets where the challenges of conducting our business can be significantly different from those we have faced in the United States and the existing markets in which we operate and where business practices may create internal control risks. We have only recently established operations outside of the United States, with the opening of an office in London. We may open additional non-U.S. offices in the future. There are a number of risks inherent in conducting international business, including:

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fluctuations in foreign currency exchange rates;



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the burden of complying with a wide variety of laws and regulatory regimes, including those relating to labor matters, consumer and data protection, privacy, network security, encryption and taxes;



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tariffs, export and import restrictions, restrictions on foreign investments, sanctions, and other trade barriers or protection measures;



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costs and difficulties of localizing our services;



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lack of acceptance of our localized services;



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difficulties in and costs of staffing, managing, and operating our international operations;



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tax issues, including restrictions on repatriating earnings and with respect to our corporate operating structure and intercompany arrangements;



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weaker intellectual property protection;



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economic weakness or currency related crises;



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limitations on the ability of our self-service sales model to attract international customers;



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our limited historical sale experience outside the United States;



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our ability to adapt to our marketing and selling efforts to different cultures and customer requirements;



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common local business behaviors that are in direct conflict with our policies and codes of conduct and ethics;



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corporate espionage; and



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political instability and security risks in the countries where we are doing business.

        If we are unable to effectively manage these risks, our relationships with our existing and prospective customers, strategic partners and employees and our operations outside of the United States may be adversely affected.

If we are not able to maintain and enhance our brand and maintain and increase market awareness of our company and services, then our business, results of operations and financial condition may be adversely affected.

        We believe that maintaining and enhancing the "SendGrid" brand identity and maintaining and increasing market awareness of our company and services is critical to achieving widespread acceptance of our platform, to strengthen our relationships with our existing customers and to our ability to attract new customers. The successful promotion of our brand will depend largely on our continued marketing efforts, our ability to continue to offer high quality services, our ability to be thought leaders in our industry and our ability to successfully differentiate our platform and services from competing products and services. For example, we rely on both algorithmic and purchased listings displayed by search engines to attract a

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significant percentage of the customers. Algorithmic listings cannot be purchased, and instead are determined and displayed solely by a set of formulas designed by the search engine. Purchased listings can be purchased by advertisers in order to attract customers to their websites, but the cost of purchased search listing advertising may increase as demand for these channels grows. If search engines on which we rely for algorithmic listings modify their algorithms in an attempt to optimize their search listings, this could result in fewer potential customers clicking through to our website, requiring us to resort to additional purchased listings or other costly advertisements to attempt to replace this traffic. If one or more search engines on which we rely for purchased listings modifies or terminates its relationship with us, or increases the amounts it charges us, our expenses could rise. Any such increases could negatively affect market awareness of our brand and our business, results of operations and financial condition.

        In addition, independent industry analysts often provide reviews of our services and competing products and services, which may significantly influence the perception of our services in the marketplace. If these reviews are negative or not as strong as reviews of our competitors' products and services, then our brand may be harmed. This may cause us to lose existing customers, decrease the number of new customers that we are able to attract or lower our pricing model, each of which would likely harm our results of operations.

        From time to time, our customers and other third parties have complained about our platform, such as complaints about our pricing and customer support or the use of our platform to transmit spam, phishing scams, website links to harmful applications or other harmful or illegal material. If we do not handle customer and other complaints effectively, then our brand and reputation may suffer, our customers may lose confidence in us and our customers may reduce or cease their use of our services. In addition, many of our customers post about and discuss Internet-based products and services, including our platform and services, on social media. Our success depends, in part, on our ability to generate positive customer feedback and minimize negative feedback on social media channels where existing and potential customers seek and share information. If actions we take or changes we make to our platform or services, particularly as our platform continues to scale, upset these customers then their online commentary could negatively affect our brand and reputation. Complaints or negative publicity about us, our platform or our services could materially and adversely impact our ability to attract and retain customers, our business, results of operations and financial condition.

        The promotion of our brand also requires us to make substantial expenditures, and we anticipate that these expenditures will increase as our market becomes more competitive and as we expand into new markets. To the extent that these activities increase revenue, this revenue still may not be enough to offset the increased expenses we incur. If we do not successfully maintain and enhance our brand, then our business may not grow, we may see our pricing power reduced relative to competitors and we may lose customers, all of which would adversely affect our business, results of operations and financial condition.

The market in which we participate is highly competitive and, if we do not compete effectively, our operating results could be harmed.

        We provide cloud-based services that enable businesses to reach their customers using email for both transactional and marketing purposes. The market for providing these services is fragmented, with some vendors addressing transactional email services, some vendors addressing email marketing services and other vendors providing a broad array of services that include transactional and marketing services as part of a software suite or broader portfolio of software offerings. Notwithstanding the availability of third-party software services, some businesses rely on internally-developed solutions for their email communications needs. The market for digital communications services is rapidly evolving, creating opportunity for new competitors to enter the market with point product solutions or addressing specific

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segments of the market. In addition, in some instances, we have strategic or other commercial relationships with companies with which we also compete, such as Amazon Web Services. Our competitors include:

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Companies that offer transactional email services, including Amazon, Mailgun, Oracle and SparkPost; and



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Companies that offer email marketing services, including Adobe, Campaign Monitor, Endurance, IBM, MailChimp, Oracle and Salesforce.

        We believe the principal factors on which we compete include: completeness of offering; credibility with developers and marketers; global reach; ease of adoption; features and functionality; platform scalability, reliability, security and performance; brand awareness and reputation; integration with third-party applications and data sources; customer support; and the total cost of deployment and ownership. Our current and potential competitors may develop and market new technologies with similar or superior functionality to our platform or at a cheaper price point that render our existing or future services less competitive or obsolete, and we may need to decrease the prices or accept less favorable terms for subscriptions for our services in order to remain competitive. If we are unable to maintain our pricing due to competitive pressures, our margins will be reduced and our operating results will be negatively affected.

        Our current and potential competitors also may have significantly more financial, technical, marketing and other resources than we have, may be able to devote greater resources to the development, promotion, sale and support of their products and services, may have more extensive customer bases and broader customer relationships and may have longer operating histories and greater name recognition. As a result, these competitors may respond faster to new technologies and undertake more extensive marketing programs for their products or services. If these companies decide to further invest in and develop, market or resell competitive products or services, or acquire or form a strategic alliance with one of our other competitors, our ability to compete effectively could be significantly compromised and our operating results could be harmed. In a few cases, these vendors may offer competitive products or services at little or no additional cost by bundling it with their existing suite of applications. To the extent any of our competitors has existing relationships with potential customers for either email software or other applications, those customers may be unwilling to purchase our platform because of their existing relationships with our competitor. If we are unable to compete with such companies, the demand for our platform and services could substantially decline.

If our security measures are breached or unauthorized access to our or our customers' and their end customers' private or proprietary data is otherwise obtained, our platform or services may be perceived as not being secure, our reputation may be severely harmed, customers may reduce the use of or stop using our services, we may incur significant liabilities and we may lose the ability to offer our customers a credit card payment option.

        Our operations involve the storage and transmission of data of our customers and their end customers, including personally identifiable information. Security breaches or other incidents could result in unauthorized access to, loss of or unauthorized disclosure of this information, litigation, indemnity obligations and other possible liabilities, as well as negative publicity, which could damage our reputation, impair our sales and harm our customers and our business. Cyberattacks and other malicious Internet-based activity continue to increase generally, and cloud-based service providers like us are regularly targeted. For example, in 2013, we suffered a distributed denial of service attack, whereby a cybercriminal gained access to our webpage and, in 2015, a cybercriminal gained unauthorized access to several of our internal systems, including those containing sensitive and confidential customer data. It is possible that we may face increased risk of cybersecurity attacks as compromising our platform may allow cybercriminals to send a large number of phishing emails that bypasses end users' spam filters. If our security measures are compromised as a result of third-party action, employee or customer error, malfeasance, stolen or fraudulently obtained log-in credentials or otherwise, our reputation could be damaged, our business may be harmed and we could incur significant liability. In addition, if the security measures of our customers

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are compromised, even without any actual compromise of our own systems, we may face negative publicity or reputational harm if our customers or anyone else incorrectly attributes the blame for such security breaches to us or our systems. We may be unable to anticipate or prevent techniques used to obtain unauthorized access or to sabotage our systems because they change frequently and generally are not detected until after an incident has occurred. As we increase our customer base and our brand becomes more widely known and recognized, we may become more of a target for third parties seeking to compromise our security systems or gain unauthorized access to our customers' data. Further, as we rely more on third-party and public-cloud infrastructure, such as Amazon Web Services, we will become more dependent on third-party security measures to protect against unauthorized access, cyberattacks and the mishandling of customer data. Any security breach, whether actual or perceived, would harm our reputation, and we could lose customers and fail to acquire new customers. In addition, many governments have enacted laws requiring companies to notify individuals of data security incidents involving certain types of personal data and any mandatory disclosures we may make regarding a security breach would likely lead to widespread negative publicity, which may cause our customers to lose confidence in the effectiveness of our data security measures. It is also possible that security breaches sustained by our competitors could result in negative publicity for our entire industry that indirectly harms our reputation and diminishes demand for our services.

        Furthermore, failure to maintain compliance with the data protection policy standards adopted by the major credit card issuers and other payment processors may limit our ability to offer our customers a credit card or online payment option. Any loss of our ability to offer our customers a credit card payment or online payment option would harm our reputation and make our platform and services less attractive to many organizations by negatively impacting our customer experience and significantly increasing our administrative costs related to customer payment processing.

        Our existing general liability insurance may not cover any, or cover only a portion of any, potential claims related to security breaches to which we are exposed or may not be adequate to indemnify us for all or any portion of liabilities that may be imposed. In addition, there can be no assurance that the limitations on liability in our contracts would be enforceable or adequate or would otherwise protect us from any such liabilities with respect to any particular claim. Any imposition of liability that is not covered by insurance or is in excess of insurance coverage would increase our operating expenses and reduce our net income.

Our customers' and other users' violation of our policies or other misuse of our platform to transmit offensive or illegal messages, phish, spam, website links to harmful applications or for other fraudulent activity could damage our reputation, and we may face liability for unauthorized, inaccurate or fraudulent information distributed via our platform.

        Despite our ongoing and substantial efforts to limit such use, a portion of our customers use our platform to transmit offensive or illegal messages, spam, phishing scams and website links to harmful applications, reproduce and distribute copyrighted material or the trademarks of others without permission, and report inaccurate or fraudulent data or information. These issues also arise with respect to a portion of those users who use our platform on a free trial basis. These actions are in violation of our policies. However, our efforts to defeat spamming attacks and other fraudulent activity will not prevent all such attacks and activity, such use of our platform could damage our reputation and we could face claims for damages, copyright or trademark infringement, defamation, negligence or fraud. Moreover, our customers' and other users' promotion of their products and services through our platform might not comply with federal, state and foreign laws. We rely on contractual representations made to us by our customers that their use of our platform will comply with our policies and applicable law, including, without limitation, our email policy. Although we retain the right to verify that customers and other users are abiding by our email policy and to review their email and email lists, our customers and other users are ultimately responsible for compliance with our policies, and we do not systematically audit our customers or other users to confirm compliance with our policies. We cannot predict whether our role in facilitating

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our customers' or other users' activities would expose us to liability under applicable law. Even if claims asserted against us do not result in liability, we may incur substantial costs in investigating and defending such claims. If we are found liable for our customers' or other users' activities, we could be required to pay fines or penalties, redesign business methods or otherwise expend resources to remedy any damages caused by such actions and to avoid future liability.

Real or perceived errors, failures, vulnerabilities or bugs in our software could adversely affect our business, results of operations, financial condition and growth prospects.

        Our platform is complex, and therefore, undetected errors, failures or bugs may occur, particularly as we continue to scale our platform to accommodate future growth. Our platform may be used in IT environments with different operating systems, system management software, applications, devices, databases and equipment and networking configurations, which may cause errors or failures of our platform or other aspects of the IT environment into which it is deployed. Despite testing by us, errors, failures or bugs may not be found until our platform is used by our customers. Real or perceived errors, failures or bugs in our platform or services could result in negative publicity, loss of or delay in market acceptance of our platform or services, weakening of our competitive position, claims by customers for losses sustained by them or failure to meet any stated service level commitments in our customer agreements. For example, in May 2017, we experienced an error with a database that stores customers' Marketing Campaigns contact information that resulted in delayed email sending for several hours. Correcting any of these problems could require significant expenditures of our capital and other resources and could cause interruptions, delays or cessation in the sale of our services, which could cause us to lose existing or potential customers and adversely affect our business, results of operations, and financial condition.

Our platform and services and our business are subject to a variety of U.S. and international laws and regulations, including those regarding privacy, data protection and information security. Any failure by us or our third-party service providers, as well as the failure of our platform or services to comply with applicable laws and regulations would harm our business, results of operations and financial condition.

        We are subject to a variety of laws and regulations, including regulation by various federal government agencies, including the U.S. Federal Trade Commission, or FTC, and state and local agencies. The United States and various state and foreign governments have adopted or proposed limitations on, or requirements regarding, the collection, distribution, use, security and storage of personally identifiable information of individuals, and the FTC and many state attorneys general are applying federal and state consumer protection laws to impose standards on the online collection, use and dissemination of data. Self-regulatory obligations, other industry standards, policies and other legal obligations may apply to our collection, distribution, use, security or storage of personally identifiable information or other data relating to individuals. In addition, the federal government and many states and foreign governments have enacted laws requiring companies to notify individuals of data security breaches involving certain types of personal data. These obligations may be interpreted and applied in an inconsistent manner from one jurisdiction to another and may conflict with one another, other regulatory requirements or our internal practices. Any failure or perceived failure by us to comply with United States, European Union or other foreign privacy or security laws, regulations, policies, industry standards or legal obligations or any security incident that results in the unauthorized access to, or acquisition, release or transfer of, personally identifiable information or other customer data may result in governmental enforcement actions, litigation, fines and penalties or adverse publicity and could cause our customers to lose trust in us, which could have an adverse effect on our reputation and business.

        We expect that there will continue to be new proposed laws, regulations and industry standards concerning privacy, data protection and information security in the United States, the European Union and other jurisdictions, and we cannot yet determine the impact such future laws, regulations and

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standards may have on our business. For example, in May 2018, the general data protection regulation, or GDPR, will come into force, bringing with it a complete overhaul of E.U. data protection laws: the new rules will supersede current E.U. data protection legislation, impose more stringent E.U. data protection requirements, and provide for greater penalties for noncompliance. Changing definitions of personal data and information may also limit or inhibit our ability to operate or expand our business, including limiting strategic partnerships that may involve the sharing of data. Also, some jurisdictions require that certain types of data be retained on servers within these jurisdictions. Our failure to comply with applicable laws, directives and regulations may result in enforcement action against us, including fines and imprisonment, and damage to our reputation, any of which may have an adverse effect on our business and operating results. Further, in October 2015, the European Court of Justice issued a ruling invalidating the U.S.-E.U. Safe Harbor Framework, which facilitated personal data transfers to the United States in compliance with applicable E.U. data protection laws. In July 2016, the European Union and the United States political authorities adopted the E.U.-U.S. Privacy Shield, or Privacy Shield, replacing the Safe Harbor Framework and providing a new mechanism for companies to transfer E.U. personal data to the United States. U.S. organizations wishing to self-certify under the Privacy Shield have to pledge their compliance with its seven core and sixteen supplemental principles, which are based on European Data Protection Law. We have assessed the requirements of Privacy Shield and are implementing changes internally in order to meet the requirements. We plan to complete this process, submit our application and become self-certified under the Privacy Shield, by the end of this year.

        We publicly post our privacy policies and practices concerning our processing, use and disclosure of the personally identifiable information provided to us by our website visitors. Our publication of our privacy policies and other statements we publish that provide promises and assurances about privacy and security can subject us to potential state and federal action if they are found to be deceptive or misrepresentative of our practices.

        If our service is perceived to cause, or is otherwise unfavorably associated with, violations of privacy or data security requirements, it may subject us or our customers to public criticism and potential legal liability. Existing and potential privacy laws and regulations concerning privacy and data security and increasing sensitivity of consumers to unauthorized processing of personal information may create negative public reactions to technologies, products and services such as ours. Public concerns regarding personal information processing, privacy and security may cause some of our customers' end users to be less likely to visit their websites or otherwise interact with them. If enough end users choose not to visit our customers' websites or otherwise interact with them, our customers could stop using our platform. This, in turn, may reduce the value of our services and slow or eliminate the growth of our business.

        Evolving and changing definitions of what constitutes "Personal Information" and "Personal Data" within the European Union, the United States and elsewhere, especially relating to the classification of internet protocol, or IP, addresses, machine or device identification numbers, location data and other information, may limit or inhibit our ability to operate or expand our business. Future laws, regulations, standards and other obligations could impair our ability to collect or use information that we utilize to provide email delivery and marketing services to our customers, thereby impairing our ability to maintain and grow our customer base and increase revenue. Future restrictions on the collection, use, sharing or disclosure of our customers' data or additional requirements for express or implied consent of customers for the use and disclosure of such information limit our ability to develop new services and features.

United States federal legislation and the laws of many foreign countries impose certain obligations on the senders of commercial emails, which could minimize the effectiveness of our platform, and establish financial penalties for non-compliance, which could increase the costs of our business.

        The Federal Controlling the Assault of Non-Solicited Pornography and Marketing Act of 2003, or the CAN-SPAM Act, establishes certain requirements for commercial email messages and transactional email messages and specifies penalties for the transmission of email messages that are intended to deceive the

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recipient as to source or content. The CAN-SPAM Act, among other things, obligates the sender of commercial emails to provide recipients with the ability to opt out of receiving future commercial emails from the sender. In addition, some states have passed laws regulating commercial email practices that are significantly more restrictive and difficult to comply with than the CAN-SPAM Act. For example, Utah and Michigan prohibit the sending of email messages that advertise products or services that minors are prohibited by law from purchasing (e.g., alcoholic beverages, tobacco products, illegal drugs) or that contain content harmful to minors (e.g., pornography) to email addresses listed on specified child protection registries. Some portions of these state laws may not be preempted by the CAN-SPAM Act. In addition, certain foreign jurisdictions, such as Australia, Canada and the European Union, have enacted laws that regulate sending email, and some of these laws are more restrictive than U.S. laws. For example, some foreign laws prohibit sending broad categories of email unless the recipient has provided the sender advance consent to receipt of such email, or in other words has "opted-in" to receiving such email. If we were found to be in violation of the CAN-SPAM Act, applicable state laws governing email not preempted by the CAN-SPAM Act or foreign laws regulating the distribution of email, whether as a result of violations by our customers or our own acts or omissions, we could be required to pay large penalties, which would adversely affect our financial condition, significantly harm our business and injure our reputation. The terms of injunctions, judgments, consent decrees or settlement agreements entered into connection with enforcement actions or investigations against our company in connection with any of the foregoing laws may also require us to change one or more aspects of the way we operate our business, which could impair our ability to attract and retain customers or could increase our operating costs.

The standards that private entities and inbox service providers use to regulate the use and delivery of email have in the past interfered with, and may in the future interfere with, the effectiveness of our platform and our ability to conduct business.

        Our customers rely on email to communicate with their existing or prospective customers. Various private entities attempt to regulate the use of email for commercial solicitation. These entities often advocate standards of conduct or practice that significantly exceed current legal requirements and classify certain email solicitations that comply with current legal requirements as spam. Some of these entities maintain "blacklists" of companies and individuals, and the websites, inbox service providers and IP addresses associated with those entities or individuals that do not adhere to those standards of conduct or practices for commercial email solicitations that the blacklisting entity believes are appropriate. If a company's IP addresses are listed by a blacklisting entity, emails sent from those addresses may be blocked if they are sent to any internet domain or internet address that subscribes to the blacklisting entity's service or uses its blacklist.

        From time to time, some of our IP addresses have become, and we expect will continue to be, listed with one or more blacklisting entities due to the messaging practices of our customers and other users. We may be at an increased risk of having our IP addresses blacklisted due to our scale and volume of email processed, compared to our smaller competitors. While the overall percentage of such email solicitations that our individual customers send may be at or below reasonable standards, the total aggregate number of all emails that we process on behalf of our customers may trigger increased scrutiny from these blacklisting entities. There can be no guarantee that we will be able to successfully remove ourselves from those lists. Because we fulfill email delivery on behalf of our customers, blacklisting of this type could undermine the effectiveness of our customers' transactional email, email marketing programs and other email communications, all of which could have a material negative impact on our business and results of operations.

        Additionally, inbox service providers can block emails from reaching their users. While we continually improve our own technology and work closely with inbox service providers to maintain our deliverability rates, the implementation of new or more restrictive policies by inbox service providers may make it more difficult to deliver our customers' emails, particularly if we are not given adequate notice of a change in

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policy or struggle to update our platform or services to comply with the changed policy in a reasonable amount of time. In addition, some inbox service providers categorize as "promotional" emails that originate from email service providers and, as a result, direct them to an alternate or "tabbed" section of the recipient's inbox. If inbox service providers materially limit or halt the delivery of our customers' emails, or if we fail to deliver our customers' emails in a manner compatible with inbox service providers' email handling or authentication technologies or other policies, or if the open rates of our customers' emails are negatively impacted by the actions of inbox service providers to categorize emails, then customers may question the effectiveness of our platform and cancel their accounts. This, in turn, would harm our business and financial condition.

We are subject to governmental export controls and economic sanctions and anti-corruption laws and regulations that could impair our ability to compete in international markets and subject us to liability if we are not in compliance with applicable laws.

        Various of our operations and services may be subject to export control and economic sanctions regulations, including the U.S. Export Administration Regulations, U.S. Customs regulations and various economic and trade sanctions regulations administered by the U.S. Treasury Department's Office of Foreign Assets Controls. The provision of our services must be made in compliance with these laws and regulations. Obtaining the necessary authorizations, including any required license, for a particular deployment may be time-consuming, is not guaranteed and may result in the delay or loss of sales opportunities. In addition, changes in our platform capabilities or services, or changes in applicable export or economic sanctions regulations may create delays in the introduction and deployment of our platform and services in international markets, or, in some cases, prevent the provision of our services to certain countries or end users. Any change in export or economic sanctions regulations, shift in the enforcement or scope of existing regulations, or change in the countries, governments, persons or technologies targeted by such regulations, could also result in decreased use of our platform and services, or in our decreased ability to provide our services to existing or prospective customers with international operations. Any decreased use of our platform and services or limitation on our ability to provide our services could adversely affect our business, results of operations and financial condition.

        Furthermore, U.S. export control laws and economic sanctions prohibit the exportation of certain products and services to countries, territories, governments and persons targeted by U.S. sanctions. While we are currently taking precautions to prevent our platform from being accessed by persons targeted by U.S. sanctions, including IP address blocking, such measures may be circumvented. Given the technical limitations in developing measures that will prevent access to internet based services from particular geographies or by particular individuals, we have previously identified and expect we will continue to identify customer accounts for our platform and services that originate or that we suspect originate from countries that are subject to U.S. embargoes.

        We are aware that trials of and subscriptions to our platform have been initiated by persons and organizations in countries that are the subject of U.S. embargoes. Our provision of services in these instances was likely in violation of trade sanctions laws. We have terminated the accounts of such persons and organizations as we have become aware of them. We filed an initial voluntary self-disclosure with the U.S. Department of Treasury's Office of Foreign Assets Control, or OFAC, in February 2016 concerning these potential violations and we filed a subsequent voluntary disclosure with OFAC concerning these potential violations in May 2017. We received and responded to questions from OFAC in July 2017. We cannot predict when OFAC will complete its review and determine whether any violations occurred or levy penalties.

        Our failure to comply with U.S. sanctions or export control laws or regulations could cause us and certain of our employees to be subject to substantial civil or criminal penalties, including the possible loss of export privileges; fines, which may be imposed on us and responsible employees or managers; and, in extreme cases, the incarceration of responsible employees or managers. Each instance in which we allow

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access to our platform may constitute a separate violation of these laws. We may also suffer reputational harm.

        Further, we incorporate encryption technology into certain of our platform functions and services. Various countries regulate the import of certain encryption technology, including through import permitting and licensing requirements, and have enacted laws that could limit our customers' ability to import our services into those countries. Encryption products and the underlying technology may also be subject to export control restrictions. Governmental regulation of encryption technology and regulation of exports of encryption products, or our failure to obtain required approval for our services, when applicable, could harm our international sales and adversely affect our revenue. Compliance with applicable regulatory requirements regarding the provision of our services, including with respect to new services, may create delays in the introduction of our services in international markets, prevent our customers with international operations from deploying our platform and using our services throughout their globally-distributed systems or, in some cases, prevent the provision of our services to some countries altogether.

        Additionally, we are subject to the Foreign Corrupt Practices Act, or FCPA, the U.K. Bribery Act and other anti-corruption, anti-bribery and anti-money laundering laws in various jurisdictions both domestic and abroad. We and our third-party business partners and intermediaries may have direct or indirect interactions with officials and employees of government agencies or state-owned or affiliated entities and may be held liable for the corrupt or other illegal activities of these third-party business partners and intermediaries, our employees, representatives, contractors, channel partners, and agents, even if we do not explicitly authorize such activities. While we have policies and procedures in place to address compliance with such laws, we cannot assure you that all of our employees and agents will not take actions in violation of our policies and applicable law, for which we may be ultimately held responsible. Any violation of the FCPA or other applicable anti-bribery, anti-corruption laws, and anti-money laundering laws could result in whistleblower complaints, adverse media coverage, investigations, loss of export privileges, severe criminal or civil sanctions, or suspension or debarment from U.S. government contracts, all of which may have an adverse effect on our reputation, business, operating results and prospects.

If we are unable to develop and release enhancements to our platform and services or new services and functionality to respond to rapid technological change in a timely and cost-effective manner, our business, operating results, and financial condition could be adversely affected.

        The market for our platform and services is characterized by rapid technological change, frequent new product and service introductions and enhancements, changing customer demands, and evolving industry standards. The introduction of products and services embodying new technologies can quickly make existing products and services obsolete and unmarketable. For example, new Internet standards and technologies or new standards in the field of operating system support could emerge that are incompatible with customer deployments of our platform. The success of any enhancements or improvements to our platform or any new services or functionality depends on several factors, including timely completion, competitive pricing, adequate quality testing, integration with existing technologies and our platform, and overall market acceptance. We cannot be sure that we will succeed in developing, marketing and delivering on a timely and cost-effective basis enhancements or improvements to our platform or any services or functionality that respond to technological change or new customer requirements, nor can we be sure that any enhancements or improvements to our platform will achieve market acceptance. For example, we introduced our Marketing Campaigns service in late 2015 and the service may not achieve broad market acceptance or be cost-effective in the long term.

        Further, our success depends on meeting customer demands and expectations both with respect to the ability of our API to integrate within their applications and the speed, reliability and effectiveness of our platform. For example, if we are unable to adapt our platform on a timely basis to new email or operating system protocols or we fail to effectively integrate our API with new technologies and standards, the ability

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of our platform to integrate with our customers' systems or satisfy our customers' needs may be impaired. In addition, because we use both co-located data centers and public cloud resources to host our platform, we need to continually enhance and improve our platform to keep pace with changes in Internet-related hardware, software, communications, and database technologies and standards. As we transition to Amazon Web Services to host a portion of our platform, we will become more dependent on third parties to keep pace with such changes. If we cannot deliver technologically competitive services, we could lose customers and our revenues and stock price may decline.

        Any new services or functionality that we develop may not be introduced in a timely or cost-effective manner, may contain errors or defects, or may not achieve the broad market acceptance necessary to generate sufficient revenue. Moreover, even if we introduce new services or functionality, we may experience a decline in revenue of our existing services that is not offset by revenue from the new services or functionality. For example, customers may delay making purchases of new services to permit them to make a more thorough evaluation of these services or until industry and marketplace reviews become widely available. In addition, we may lose existing customers that choose a competitor's products and services or that choose to rely on an internally-developed solution, rather than migrate to our new services. Similarly, if we offer a bundle of our services that our customers and prospective customers do not find compelling, our business will be harmed. This could result in a temporary or permanent revenue shortfall and adversely affect our business.

We may not be able to scale our technology infrastructure and platform quickly enough to meet our customers' growing needs and if we are not able to grow efficiently, our operating results could be harmed.

        As usage of our platform grows, we will need to continue making significant investments to develop and implement new technologies in our infrastructure operations to support our growth. In addition, we will need to appropriately scale our internal business systems and our services organization, including basic customer support and our Expert Services team, to serve our growing customer base. Any failure of, or delay in, these efforts could impair the performance of our platform and reduce customer satisfaction and reduce our growth. Even if we are able to upgrade our systems and expand our staff, any such expansion may be expensive and complex. To the extent that we do not effectively scale our platform and infrastructure to meet the growing needs of our customers or are not able to manage that growth effectively, we may not be able to grow as quickly as we anticipate, our customers may reduce or stop use of our services, we may be unable to compete effectively and our business and operating results may be harmed.

We are primarily dependent on a single service, our Email API service, and the lack of continued market acceptance of our Email API service could cause our operating results to suffer.

        Sales of our Email API service account for a substantial majority of our revenue. We expect that we will be substantially dependent on our Email API service to generate revenue for the foreseeable future. As a result, our operating results could suffer due to:

•

any decline in demand for our Email API service;



•

the failure of our Email API service to achieve continued market acceptance;



•

the market for our Email API service not continuing to grow, or growing more slowly than we expect;



•

the introduction or increase in popularity of products and technologies that serve as a replacement or substitute for, or represent an improvement over, our Email API service;



•

technological innovations or new standards that our Email API service does not address;



•

sensitivity to current or future prices offered by us or competing solutions;

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•

our inability to release enhanced versions of our Email API service on a timely basis; and



•

the continued use and ubiquity of email in commercial transactions and applications.

        If the market for our Email API service grows more slowly than anticipated or if demand for our Email API service does not grow as quickly as anticipated, we may not be able to grow our revenue.

Our services are sold on a short-term basis, such that subscription renewal or usage rates can decrease rapidly, and if we do not accurately predict these rates, our future revenue and operating results may be harmed.

        We charge our customers based on their use of our services. We generally enter into monthly subscription agreements with our customers and, therefore, most of our customers may reduce or cease their use of our services on a monthly basis without penalty or termination charges. Our retention of customers and our customers' use of our services are used to determine our subscription net dollar retention rate. We believe subscription net dollar retention rate is an important measure for evaluating our business. Additionally, our subscription agreements for use of our Email API service include a fixed fee that covers a specified number of email credits in the applicable month. If our customers lower the number of email credits they purchase in any month, then the fixed fees we charge will decrease. We have historically experienced customer turnover as a result of many of our customers being SMBs in the entrepreneurial stage of their development that are more susceptible than larger businesses to general economic conditions and other risks affecting their businesses. We cannot accurately predict customers' usage levels on a monthly basis and the loss of customers or reductions in their usage levels of our services may each have a negative impact on our business, results of operations and financial condition, including our subscription net dollar retention rate and our subscription gross dollar churn rate, which is a component of our subscription net dollar retention rate.

Any failure to offer high-quality support may harm our relationships with our customers and have a negative impact on our business and financial condition.

        Our customers depend on our customer support team to resolve technical and operational issues relating to our platform. Our ability to provide effective customer support is largely dependent on our ability to attract, train, and retain qualified personnel with experience in supporting customers on platforms such as ours. Our customer base has grown significantly and that has and will put additional pressure on our customer support team. We may be unable to respond quickly enough to accommodate short-term increases in customer demand for support. We also may be unable to modify the scope and delivery of our support to compete with changes in the support provided by our competitors. Increased customer demand for support, without corresponding revenue, could increase costs and negatively affect our operating results. In addition, as we continue to grow our operations and expand internationally, we need to be able to provide efficient customer support that meets our customers' needs globally at scale and our customer support team will face additional challenges, including those associated with delivering support and documentation in languages other than English. If we are unable to provide efficient customer support globally at scale, our ability to grow our operations may be harmed and we may need to hire additional support personnel, which could negatively impact our operating results. In addition, we provide self-service support resources to our customers, such as our best practice guides and webcasts hosted on our website. We also rely on our user community to serve as a resource for questions on any part of our platform. Members of our user community are not obligated to participate in discussions with other users, and to the extent they do not our customers' ability to find answers to questions about our platform of services may suffer. If we are unable to develop self-service support resources that are easy to use and that our customers utilize to resolve their technical issues or if our customers choose not to take advantage of these self-service support services, customers may continue to direct support requests to our customer support team instead of relying on our self-service support resources and our customers' experience with our platform may be negatively impacted. Any failure to maintain high-quality support, or a market

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perception that we do not maintain high-quality support, could harm our reputation, our ability to sell our services to existing and prospective customers, and our business, operating results, and financial condition.

To deliver our services, we rely on internet service providers for network connectivity and co-location facilities and public cloud infrastructure to host our platform.

        We interconnect with internet service providers around the world to enable the use of our platform by our customers and we expect that we will continue to rely heavily on internet service providers for network connectivity going forward. Our reliance on internet service providers reduces our control over quality of service and exposes us to potential service outages and rate fluctuations. If a significant portion of our internet service providers stop providing us with access to their network infrastructure, fail to provide access on a cost-effective basis, cease operations, or otherwise terminate access, the delay caused by qualifying and switching to other internet service providers could be time consuming and costly and could adversely affect our business, results of operations and financial condition.

        In addition, we currently serve the majority of our platform infrastructure from multiple co-located third-party data centers located throughout the United States and in certain international locations. We are in the process of transitioning the hosting of a portion of our platform infrastructure to Amazon Web Services. As a result, our operations depend, in part, on the ability of third parties to protect these facilities and resources against damage or interruption from natural disasters, power or network failures, criminal acts and similar events. In the event that any of our hosting arrangements are terminated, or if there is a lapse of service or damage to a facility or other resource, we could experience interruptions in our platform as well as delays and additional expenses in arranging new facilities and resources. We have experienced and expect that we will in the future experience such interruptions from time to time.

        Any damage to, or failure of, the systems of our third-party providers could result in interruptions to and may cause errors or poor quality communications with our platform. Even with current and planned disaster recovery arrangements, our business could be harmed. Also, in the event of damage or interruption, our insurance policies may not adequately compensate us for any losses that we may incur. These factors in turn could further reduce our revenue, subject us to liability, damage our reputation and cause customers to become dissatisfied and fail to renew their subscriptions or demand credits, any of which could materially and adversely affect our business.

If we are unable to hire, retain and motivate qualified personnel, our business will suffer.

        Our future success depends, in part, on our ability to continue to attract and retain highly skilled personnel. From time to time we have experienced difficulty in hiring and retaining employees who have appropriate qualifications, and we are continuing our efforts to meet our hiring goals. We believe that there is, and will continue to be, intense competition for highly skilled executive, technical, marketing and other personnel with experience in the locations where we maintain offices. We must provide competitive compensation packages and a high-quality work environment to hire, retain and motivate employees. If we are unable to retain and motivate our existing employees and attract qualified personnel to fill key positions, we may be unable to manage our business effectively, including the development, marketing and sale of our services, which could adversely affect our business, results of operations and financial condition. To the extent we hire personnel from competitors, we also may be subject to allegations that they have been improperly solicited or divulged proprietary or other confidential information.

        Volatility in, or lack of performance of, our stock price may also affect our ability to attract and retain key personnel. Many of our key personnel are, or will soon be, vested in a substantial amount of shares of common stock or stock awards. Employees may be more likely to terminate their employment with us if the shares they own or the shares underlying their vested options have significantly appreciated in value relative to the original purchase prices of the shares or the exercise prices of the options, or, conversely, if the exercise prices of the options that they hold are significantly above the trading price of our common

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stock. Additionally, if our stock price underperforms or the outlook for our stock is negative, we may not be able to attract qualified personnel to join our business. Candidates making employment decisions, particularly in high-technology industries, often consider the value and growth potential of any equity they may receive in connection with their employment. As a result, any significant decline in the market price of our common stock may adversely affect our ability to attract or retain highly skilled executive, technical, marketing and other personnel. If we are unable to retain and attract highly-skilled employees, our business, results of operations and financial condition could be adversely affected.

We have experienced losses in the past, and we may not achieve or sustain profitability in the future.

        We generated net losses of $13.0 million, $5.9 million and $3.9 million in 2014, 2015 and 2016, respectively, and a net loss of $4.7 million in the nine months ended September 30, 2017. As of September 30, 2017, we had an accumulated deficit of $48.3 million. We will need to generate and sustain increased revenue levels in future periods in order to achieve or sustain profitability. We also expect our costs to increase in future periods, which could negatively affect our future operating results if our revenue does not increase commensurately. For example, we intend to continue to expend significant funds to develop and enhance our technical infrastructure, platform and services, expand our research and development efforts and selling and marketing operations, including through increased headcount, meet the increased compliance requirements associated with our transition to and operation as a public company, and expand into new markets. Our efforts to grow our business may be more costly than we expect, and we may not be able to increase our revenue enough to offset our higher operating expenses. We may incur significant losses in the future for a number of reasons, including the other risks described in this prospectus, and unforeseen expenses, difficulties, complications and delays and other unknown events. If we are unable to achieve or sustain profitability, our stock price may significantly decrease.

We may require additional financing in the future and may not be able to obtain such financing on favorable terms, if at all, which could slow or stop our ability to grow or otherwise harm our business.

        We intend to continue to make investments to support our business growth and may require additional funds to respond to business challenges, including the need to develop new features or enhance our platform, improve our operating infrastructure or acquire complementary businesses and technologies. To date, we have financed our operations primarily through our operations, private placements of our equity securities and debt financing provided by financial institutions. We may not be able to obtain additional financing on terms favorable to us, if at all. We may need to engage in equity or debt financings to secure additional funds. If we raise additional equity financing, our stockholders may experience significant dilution of their ownership interests and the per share value of our common stock could decline. If we engage in debt financing, the holders of debt would have priority over the holders of our common stock, and we may be required to accept terms that restrict our ability to incur additional indebtedness and our ability to operate our business. We may also be required to take other actions that would otherwise be in the interests of the debt holders and force us to maintain specified liquidity or other ratios, any of which could harm our business, financial condition and operating results. If we are unable to obtain adequate financing or financing on terms satisfactory to us when we require it, our ability to continue to support our business growth and to respond to business challenges could be significantly impaired, and our business may be adversely affected.

Our future quarterly results may fluctuate significantly, and if we fail to meet the expectations of analysts or investors, our stock price and the value of your investment could decline substantially.

        Our results of operations, including our revenue, operating expenses and cash flows have fluctuated from quarter to quarter in the past and may continue to fluctuate as a result of a variety of factors, many of which are outside of our control, may be difficult to predict and may or may not fully reflect the underlying performance of our business. Additionally, we have a limited operating history with the current scale of our

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business, which makes it difficult to forecast our future results. Some of the factors that may cause our results of operations to fluctuate from quarter to quarter include:

•

our ability to attract new customers;



•

our ability to retain customers and expand their usage of our services;



•

changes in customers' budgets and in the timing of their purchasing decisions, including as a result of any changes in their usage or expected usage of our services;



•

potential customers opting for alternative solutions, including internally-developed and maintained solutions, and competitive email or digital communication solutions;



•

our ability to control costs, including our operating expenses;



•

the timing and success of new products, features and services by us and our competitors;



•

changes in the competitive dynamics of our industry, including price competition or consolidation among competitors, customers or strategic partners;



•

significant security breaches of, technical difficulties with, or interruptions to, the delivery and use of our platform;



•

reputational harm or other liabilities resulting from the use of our platform to transmit spam, phishing scams, website links to harmful applications or other harmful or illegal material;



•

the collectability of receivables from customers, which may be hindered or delayed if these customers experience financial distress;



•

general economic conditions, both domestically and internationally, as well as economic conditions specifically affecting industries in which our customers participate;



•

fluctuations in foreign currency exchange rates which could, for example, cause our subscription agreements denominated in U.S. dollars to be more expensive internationally;



•

sales tax and other tax determinations by authorities in the jurisdictions in which we conduct business;



•

the impact of new accounting pronouncements; and



•

fluctuations in stock-based compensation expense.

        The occurrence of one or more of the foregoing and other factors may cause our results of operations to vary significantly. Accordingly, our results of operations in any one quarter may not be meaningful and should not be relied upon as indicative of future performance. Additionally, if our quarterly results of operations fall below the expectations of investors or securities analysts who follow our stock, the price of our common stock could decline substantially, and we could face costly lawsuits, including securities class action suits.

Our revenue is affected by seasonality.

        Our revenue is directly correlated with the number of emails sent by our customers. During the fourth-quarter holiday season, our customers process a relatively higher number of transactions, run a greater number of marketing programs, and therefore send more emails. Partially because of this seasonality, a greater percentage of our annual revenue has come from our fourth quarter than from other quarters. In 2014, 2015 and 2016, 28% to 29% of our annual revenue was in our fourth quarter. While we believe that this seasonality has affected and will continue to affect our quarterly results, our rapid growth has largely masked seasonal trends to date. Seasonality may become more pronounced in the future to the extent our revenue mix shifts between our Email API, Marketing Campaigns and other revenue sources

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over time. As a result, historical patterns in our business may not be a reliable indicator of our future performance.

Our reliance on cloud-based technologies from third parties may adversely affect our business, results of operations and financial condition.

        We rely heavily on hosted cloud-based technologies from third parties in order to operate critical internal functions of our business, including our accounting, billing and office management functions. If these services become unavailable due to extended outages or interruptions, or because they are no longer available on commercially reasonable terms or prices, our expenses could increase. As a result, our ability to manage our operations could be interrupted and our processes for managing our sales process and supporting our customers could be impaired until equivalent services, if available, are identified, obtained and implemented, all of which could adversely affect our business, results of operations and financial condition.

Our corporate culture has contributed to our success, and if we cannot maintain this culture as we grow, we could lose the innovation, creativity and teamwork fostered by our culture, which could harm our business.

        We believe that our culture has been and will continue to be a key contributor to our success. Between December 31, 2011 and September 30, 2017, we have increased the size of our full-time workforce by 350 employees, and we intend to continue to hire aggressively as we expand. If we do not continue to maintain our corporate culture as we grow, we may be unable to foster the innovation, creativity and teamwork we believe we need to support our growth. Moreover, many of our employees may be able to receive significant proceeds from sales of our common stock in the public markets after this offering, which could lead to disparities of wealth among our employees that adversely affects relations among employees and our culture in general. Our substantial anticipated headcount growth and our transition from a private company to a public company may result in a change to our corporate culture, which could harm our business.

Acquisition opportunities could be difficult to identify, and any pursued acquisitions could divert the attention of management, pose integration challenges, disrupt our business, dilute stockholder value and adversely affect our operating results and financial condition.

        We have in the past acquired and may in the future seek to acquire or invest in businesses, products or technologies that we believe could complement or expand our platform, enhance our technical capabilities or otherwise offer growth opportunities. For example, in 2017, we acquired JCKM, Inc., or Bizzy, a marketing automation company. We do not know if we will be able to fully realize the anticipated benefits of this or other acquisitions and we may not be successful in integrating acquired technologies and businesses into our company. Acquisitions may disrupt our business, divert our resources and require significant management attention that would otherwise be available for development of our existing business.

        We also may not achieve the anticipated benefits from an acquired business due to a number of factors, including:

•

inability to integrate or benefit from acquired technologies or services in a profitable manner;



•

unanticipated costs, accounting charges or other liabilities associated with the acquisition;



•

incurrence of acquisition-related costs;



•

difficulty integrating the accounting systems, operations and personnel of the acquired business, including due to language, geographical or cultural differences;

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•

difficulties and additional expenses associated with supporting legacy products and hosting infrastructure of the acquired business;



•

difficulty converting the customers of the acquired business onto our platform and contract terms, including disparities in the revenues, licensing, support or professional services model of the acquired company;



•

adverse effects to our existing business relationships with business partners and customers as a result of the acquisition;



•

the potential loss of key employees;



•

retention of employees from the acquired company;



•

use of resources that are needed in other parts of our business;



•

litigation or other claims arising in connection with the acquired company;



•

additional regulatory and compliance requirements; and



•

use of substantial portions of our available cash to consummate the acquisition.

        In addition, a significant portion of the purchase price of companies we acquire may be allocated to goodwill and other intangible assets, which must be assessed for impairment at least annually. Acquisitions could also result in dilutive issuances of equity securities or the incurrence of debt, which could adversely affect our operating results. In the future, if our acquisitions do not yield expected returns, we may be required to take charges to our operating results based on this impairment assessment process, which could adversely affect our results of operations.

If the estimates and assumptions we have used to calculate the size of our total addressable target market are inaccurate, our future growth rate may be limited.

        We have estimated the size of our total addressable target market based on data published by third parties and on internally generated data and assumptions. While we believe our market size information is generally reliable, such information is inherently imprecise, and relies on our and third parties' projections, assumptions and estimates within our target market, which are necessarily subject to a high degree of uncertainty and risk due to a variety of factors, including those described in this prospectus. If third-party or internally generated data prove to be inaccurate or we make errors in our assumptions based on that data, including how current customer data and trends may apply to potential future customers and the number and type of potential customers, our future growth rate may be lower than what we currently estimate. In addition, these inaccuracies or errors may cause us to misallocate capital and other business resources, which could divert resources from more valuable alternative projects and harm our business.

        Even if our target market meets our size estimates, we may not grow our business. Our growth is subject to many factors, including our success in expanding our international operations, continuing to promote the use of our services by our customers for their marketing needs and otherwise implementing our business strategy, which are subject to many risks and uncertainties. Accordingly, the information regarding the size of our total addressable market included in this prospectus should not be taken as indicative of our future growth.

Our platform or services may infringe the intellectual property rights of third parties and this may create liability for us or otherwise harm our business.

        From time to time third parties have claimed, and may in the future claim, that our platform capabilities or services infringe their intellectual property rights, and such claims may also result in claims against our customers. These claims may damage our brand and reputation, harm our customer relationships and create liability for us. We expect the number of such claims will increase as the number of

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products and services and the level of competition in our market grows, the functionality of our platform or services overlap with that of other products and services, and the volume of issued software patents and patent applications continues to increase.

        Companies in the software and technology industries, including some of our current and potential competitors, own large numbers of patents, copyrights, trademarks, and trade secrets and frequently enter into litigation based on allegations of infringement or other violations of intellectual property rights. In addition, many of these companies have the capability to dedicate substantially greater resources to enforce their intellectual property rights and to defend claims that may be brought against them. Furthermore, patent holding companies, non-practicing entities and other adverse patent owners that are not deterred by our existing intellectual property protections have asserted, and in the future may seek to assert, patent claims against us. From time to time, third parties have contacted us inviting us to license their patents and may, in the future, assert patent, copyright, trademark or other intellectual property rights against us, our strategic partners or our customers. We have received, and may in the future receive, notices that claim we have misappropriated, misused, or infringed other parties' intellectual property rights, and, to the extent we gain greater market visibility, we face a higher risk of being the subject of intellectual property infringement claims, which is not uncommon with respect to our market.

        There may be third-party intellectual property rights, including issued or pending patents, that cover significant aspects of our technologies or business methods. In addition, if we acquire or license technologies from third parties, we may be exposed to increased risk of being the subject of intellectual property infringement due to, among other things, our lower level of visibility into the development process with respect to such technology and the care taken to safeguard against infringement risks. Any intellectual property claims, with or without merit, may be very time-consuming, may be expensive to settle or litigate, and may divert our management's attention and other resources. These claims may also subject us to significant liability for damages, potentially including treble damages if we are found to have willfully infringed patents or copyrights, and may require us to indemnify our customers for liabilities they incur as a result of such claims. These claims may also result in our having to stop using technology found to be in violation of a third party's rights. We may be required to seek a license for the intellectual property, which may not be available on reasonable terms or at all. Even if a license were available, we may be required to pay significant royalties, which would increase our operating expenses. Alternatively, we may be required to develop alternative non-infringing technology, which may require significant time, effort, and expense, and may affect the performance or features of our platform. If we cannot license or develop alternative non-infringing substitutes for any infringing technology used in any aspect of our business, we would be forced to limit or stop sales of our services and may be unable to compete effectively. Any of these results would adversely affect our business, operating results and financial condition.

Indemnity provisions in various agreements potentially expose us to substantial liability for intellectual property infringement and other losses.

        Our agreements with customers and other third parties typically include indemnification or other provisions under which we agree to indemnify or otherwise be liable to them for losses suffered or incurred as a result of claims of intellectual property infringement or other liabilities relating to or arising from our platform or services or other acts or omissions. Large indemnity payments or damage claims from contractual breach could harm our business, results of operations and financial condition. Although we normally contractually limit our liability with respect to such obligations, we may still incur substantial liability related to them. Any dispute with a customer with respect to such obligations could have adverse effects on our relationship with that customer and other current and prospective customers, reduce demand for our platform and adversely affect our business, results of operations and financial condition.

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Our platform contains third-party open source software components, and failure to comply with the terms of the underlying open source software licenses could restrict our ability to sell our services.

        Our platform incorporates open source software code. An open source license allows the use, modification and distribution of software in source code form. Certain kinds of open source licenses further require that any person who creates a product or service that contains, links to, or is derived from software that was subject to an open source license must also make their own product or service subject to the same open source license. Using software that is subject to this kind of open source license can lead to a requirement that our services be provided free of charge or be made available or distributed in source code form. Although we do not believe our platform includes any open source software in a manner that would result in the imposition of any such requirement, the interpretation of open source licenses is legally complex and, despite our efforts, it is possible that our platform could be found to contain this type of open source software. Further, few of the licenses applicable to open source software have been interpreted by courts, and there is a risk that these licenses could be construed in a manner that could impose unanticipated conditions or restrictions on our ability to commercialize our platform or services in the future.

        Moreover, we cannot assure you that our processes for controlling our use of open source software in our platform will be effective. If we have not complied with the terms of an applicable open source software license, we could be required to seek licenses from third parties to continue offering our platform on terms that are not economically feasible, to re-engineer our services to remove or replace the open source software, to discontinue the sale of our services if re-engineering could not be accomplished on a timely basis, to pay monetary damages, or to make generally available the source code for our proprietary technology, any of which could adversely affect our business, operating results, and financial condition.

        In addition to risks related to license requirements, use of open source software can involve greater risks than those associated with use of third-party commercial software, as open source licensors generally do not provide warranties or assurance of title, performance, non-infringement, or controls on origin of the software. There is typically no support available for open source software, and we cannot assure you that the authors of such open source software will not abandon further development and maintenance. Many of the risks associated with the use of open source software, such as the lack of warranties or assurances of title or performance, cannot be eliminated, and could, if not properly addressed, negatively affect our business. We have established processes to help alleviate these risks, including a review process for screening requests from our development organizations for the use of open source software, but we cannot be sure that all open source software is identified or submitted for approval prior to use in our platform.

        Responding to any infringement claim, regardless of its validity, or discovering open source software code in our platform could harm our business, operating results and financial condition, by, among other things:

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resulting in time-consuming and costly litigation;



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diverting management's time and attention from developing our business;



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requiring us to pay monetary damages or enter into royalty and licensing agreements on terms that may not be favorable to us;



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causing delays in the deployment of our platform;



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requiring us to stop selling some aspects of our platform;



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requiring us to redesign certain components of our platform using alternative non-infringing or non-open source technology or practices, which could require significant effort and expense;



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requiring us to disclose our software source code, the detailed program commands for our software; and



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requiring us to satisfy indemnification obligations to our customers.

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Any failure to protect our proprietary technology and intellectual property rights could substantially harm our business and operating results.

        Our success and ability to compete depend in part on our ability to protect our proprietary technology and intellectual property. To safeguard these rights, we rely on a combination of patent, trademark, copyright and trade secret laws and contractual protections in the United States and other jurisdictions, all of which provide only limited protection and may not now or in the future provide us with a competitive advantage.

        In order to protect our unpatented proprietary technologies and processes, we rely on trade secret laws and confidentiality and invention assignment agreements with our employees, consultants, strategic partners, vendors and others. Also, despite our efforts to protect our proprietary technology and trade secrets, unauthorized parties may attempt to misappropriate, copy, reverse engineer or otherwise obtain and use them. In addition, others may independently discover our trade secrets, in which case we would not be able to assert trade secret rights, or develop similar technologies and processes. Further, the contractual provisions that we enter into may not prevent unauthorized use or disclosure of our proprietary technology or intellectual property rights and may not provide an adequate remedy in the event of unauthorized use or disclosure of our proprietary technology or intellectual property rights. Moreover, policing unauthorized use of our technologies, trade secrets and intellectual property is difficult, expensive and time-consuming, particularly in foreign countries where the laws may not be as protective of intellectual property rights as those in the United States and where mechanisms for enforcement of intellectual property rights may be weak. To the extent that we expand our activities outside of the United States, our exposure to unauthorized copying and use of our platform and proprietary information may increase. We may be unable to determine the extent of any unauthorized use or infringement of our platform, technologies or intellectual property rights.

        We cannot assure you that any patents we have obtained or that may issue to us in the future will give us the protection that we seek or that such patents will not be challenged, invalidated or circumvented. As of September 30, 2017, we had one issued patent. Any patents that may issue to us in the future may not provide sufficiently broad protection and may not be enforceable in actions against alleged infringers. Obtaining and enforcing software patents in the United States is becoming increasingly challenging. Any patents we have obtained or may obtain in the future may be found to be invalid or unenforceable in light of recent and future changes in the law. Similarly, we cannot assure you that any future trademark registrations will be issued from current or future applications or that any registered trademarks we have obtained or may obtain in the future will be enforceable or provide adequate protection of our proprietary rights. We also license software from third parties for integration into our software, including open source software and other software available on commercially reasonable terms. We cannot assure you that such third parties will maintain such software or continue to make it available.

        There can be no assurance that the steps that we take will be adequate to protect our proprietary technology and intellectual property, that others will not develop or patent similar or superior technologies, products or services, or that our trademark, patent and other intellectual property rights will not be challenged, invalidated or circumvented by others. Furthermore, effective trademark, patent, copyright and trade secret protection may not be available in every country in which our software is available or where we have employees or independent contractors. In addition, the legal standards relating to the validity, enforceability, and scope of protection of intellectual property rights in internet and software-related industries are uncertain and still evolving.

        In order to protect our proprietary technology and intellectual property rights, we may be required to spend significant resources to monitor and protect these rights. Litigation brought to protect and enforce our intellectual property rights could be costly, time-consuming and distracting to management and could result in the impairment or loss of portions of our intellectual property. Furthermore, our efforts to enforce our proprietary technology and intellectual property rights may be met with defenses,

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counterclaims and countersuits attacking the validity and enforceability of our intellectual property rights. Our failure to secure, protect and enforce our intellectual property rights could materially and adversely affect our business.

We face exposure to foreign currency exchange rate fluctuations, and such fluctuations could adversely affect our business, results of operations and financial condition.

        As our international operations expand beyond our current London location, our exposure to the effects of fluctuations in currency exchange rates grows. While we have primarily transacted with customers and business partners in U.S. dollars, we expect to expand the number of transactions with customers and business partners that are denominated in foreign currencies in the future as we expand our business internationally. We incur expenses for some of our internet service provider costs outside of the United States in local currencies and for employee compensation and other operating expenses at our U.K. location in British pounds. Fluctuations in the exchange rates between the U.S. dollar and other currencies could result in an increase to the U.S. dollar equivalent of such expenses.

        Accordingly, changes in the value of foreign currencies relative to the U.S. dollar may, particularly in the future as we expand our business internationally, affect our results of operations due to transactional and translational remeasurements. As a result of such foreign currency exchange rate fluctuations, it could be more difficult to detect underlying trends in our business and results of operations. In addition, to the extent that fluctuations in currency exchange rates cause our results of operations to differ from our expectations or the expectations of our investors and securities analysts who follow our stock, the trading price of our common stock could be adversely affected.

        Our subscription agreements are denominated in U.S. dollars, and therefore our revenue currently is not subject to foreign currency risk. However, a strengthening of the U.S. dollar could increase the real cost of our platform to current and prospective customers outside of the United States, adversely affecting our business, results of operations and financial condition.

        We do not currently maintain a program to hedge transactional exposures in foreign currencies. However, in the future, we may use derivative instruments, such as foreign currency forward and option contracts, to hedge certain exposures to fluctuations in foreign currency exchange rates. The use of such hedging activities may not offset any or more than a portion of the adverse financial effects of unfavorable movements in foreign exchange rates over the limited time the hedges are in place. Moreover, the use of hedging instruments may introduce additional risks if we are unable to structure effective hedges with such instruments.

Taxing authorities may successfully assert that we should have collected or paid, or in the future should collect or pay, sales, use, value added or similar taxes, and we could be subject to liability with respect to past or future sales, which could adversely affect our financial results.

        We do not collect sales, use, value added and similar taxes in all jurisdictions in which we have sales, based on our belief that such taxes are not applicable. Sales, use, value added and similar tax laws and rates vary greatly by jurisdiction. Certain jurisdictions in which we do not collect such taxes may assert that such taxes are applicable, which could result in tax assessments, penalties and interest, and we may be required to collect and/or remit such taxes in the future. While we reserve estimated amounts with respect to such taxes on our financial statements, we cannot be certain that we have made sufficient reserves to cover such tax liabilities or such taxes. If our reserve amounts are inadequate, such tax assessments, penalties and interest or future requirements may adversely affect our results of operations and financial condition. Additionally, the application of federal, state, local and international tax laws to cloud services are continuously evolving. Income, sales, use or other tax laws, statutes, rules, regulations or ordinances could be enacted or amended at any time, possibly with retroactive effect, which could increase our taxes and ultimately have a negative impact on our results of operations and cash flows.

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Our ability to use our net operating losses to offset future taxable income may be subject to certain limitations.

        As of December 31, 2016, we had net operating loss, or NOL, carryforwards for U.S. federal income tax purposes of approximately $33.9 million, which expire in various years beginning in 2029 if not utilized. In general, under Section 382 of the United States Internal Revenue Code of 1986, as amended, or the Code, a corporation that undergoes an "ownership change" is subject to limitations on its ability to utilize its pre-ownership change NOLs to offset future taxable income. We are currently in the process of completing a study under Section 382 of the Code to determine the impact that ownership changes during and prior to the year ended December 31, 2016, if any, may have had on our NOLs and expect to complete the analysis within the next twelve months. As a result of this analysis, we may have an adjustment in our available NOLs recorded at December 31, 2016. If our existing NOLs are subject to limitations arising from previous ownership changes, or if we undergo an ownership change in connection with or after this offering, our ability to utilize pre-change NOLs could be further limited by Section 382. Future changes in our stock ownership, some of which are outside of our control, could also result in an ownership change under Section 382 of the Code. We will be unable to use our NOLs if we do not attain profitability sufficient to offset such available NOLs prior to their expiration. Furthermore, our ability to utilize NOLs of companies that we may acquire in the future may be subject to limitations. There is also a risk that due to regulatory changes, such as suspensions on the use of NOLs, or other unforeseen reasons, our existing NOLs could expire or otherwise be unavailable to offset future taxable income. For these reasons, we may not be able to utilize a portion of the NOLs reflected on our balance sheet, even if we attain profitability.

The enactment of legislation implementing changes in U.S. taxation of international business activities or the adoption of other tax reform policies could materially impact our financial condition and results of operations.

        U.S. tax laws, including limitations on the ability of taxpayers to claim and utilize foreign tax credits and the deferral of certain tax deductions until earnings outside of the United States are repatriated to the United States, as well as changes to U.S. tax laws that may be enacted in the future, could impact the tax treatment of our foreign earnings. Due to the anticipated expansion of our international business activities, any changes in the U.S. taxation of such activities may increase our worldwide effective tax rate and adversely affect our financial condition and results of operations.

We could be subject to additional tax liabilities.

        We are subject to federal, state and local taxes in the United States and the United Kingdom. Significant judgment is required in evaluating our tax positions and our worldwide provision for taxes. During the ordinary course of business, there are many activities and transactions for which the ultimate tax determination is uncertain. In addition, our tax obligations and effective tax rates could be adversely affected by changes in the relevant tax, accounting and other laws, regulations, principles and interpretations, including those relating to income tax nexus, by our earnings being lower than anticipated in jurisdictions where we have lower statutory rates and higher than anticipated in jurisdictions where we have higher statutory rates, by changes in foreign currency exchange rates, or by changes in the valuation of our deferred tax assets and liabilities. We may be audited in various jurisdictions, and such jurisdictions may assess additional taxes against us. Although we believe our tax estimates are reasonable, the final determination of any tax audits or litigation could be materially different from our historical tax provisions and accruals, which could have a material adverse effect on our operating results or cash flows in the period or periods for which a determination is made.

Unfavorable conditions in our industry or the global economy or reductions in marketing information technology spending could limit our ability to grow our business and negatively affect our results of operations.

        Our results of operations may vary based on the impact of changes in our industry or the global economy on us or our customers. The revenue growth and potential profitability of our business depend on demand for email services and email marketing generally and for our services in particular. Current or

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future economic uncertainties or downturns could adversely affect our business and results of operations. Negative conditions in the general economy both in the United States and abroad, including conditions resulting from changes in gross domestic product growth, financial and credit market fluctuations, political turmoil, natural catastrophes, warfare and terrorist attacks on the United States, Europe, the Asia Pacific region or elsewhere, could cause a decrease in business investments, including spending on information technology, and negatively affect the growth of our business. To the extent our platform or services are perceived by customers and potential customers as discretionary, there may be delays or reductions in spending on our services, we may need to reduce our prices in order to remain competitive and our revenue may decline. In addition, the increased pace of consolidation in certain industries may result in reduced overall spending on our platform and services. We cannot predict the timing, magnitude or duration of any economic slowdown, instability or recovery, generally or within any particular industry. If the economic conditions of the general economy or markets in which we operate worsen, our business, results of operations and financial condition could be adversely affected.

        Furthermore, many of our customers are SMBs. These organizations frequently have limited budgets and are often resource- and time-constrained and, as a result, may be significantly more affected by economic downturns than their larger, more established counterparts. As a result, SMBs may choose to spend the limited funds that they have on items other than our services. Moreover, if SMBs experience economic distress, they may be unwilling or unable to expend time and financial resources on email, which could negatively affect the overall demand for our platform, increase customer attrition and cause our revenue growth or operating results to decline.

The success of our business depends on the continued growth and acceptance of email as a communications tool and the related accessibility, expansion and reliability of the Internet infrastructure.

        The future success of our business depends on the continued widespread use of email as a primary means of communication and as the document of record for many digital business transactions. Security problems such as viruses, worms and other malicious programs, reliability issues arising from outages and damage to the Internet infrastructure or an overabundance of spam could create the perception that email is not a safe, reliable or efficient means of communication, which would discourage businesses and consumers from using email. In addition, alternative communications tools such as social media or text messaging have gained significant widespread acceptance and may gain additional acceptance for both email marketing and transactional record purposes. To the extent use of these alternative communications tools continues to grow, these tools could be used by our customers as a substitute for our services, and demand for our services and our revenues could decline.

        Furthermore, federal, state or foreign government bodies or agencies have in the past adopted, and may in the future adopt, laws or regulations affecting the use of the Internet as a commercial medium. In addition, government agencies or private organizations have imposed and may impose additional taxes, fees or other charges for accessing the Internet or commerce conducted via the Internet. We provide our platform through the Internet and many of our customers use our services in connection with their own online transactions. Changes in these laws or regulations could require us to modify our platform in order to comply with these changes and could impact our customers' businesses, reducing demand for our services. These laws or charges could limit the growth of Internet-related commerce or the use of Internet communications such as email, or result in reductions in the demand for Internet-based services such as our platform. In addition, the use of the Internet and email as business tools could be adversely affected due to delays in the development or adoption of new standards and protocols to handle increased demands in Internet activity, security, reliability, cost, ease-of-use, accessibility and quality of service. Any decrease in the use of the Internet or email would reduce demand for our platform and harm our business.

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If we experience excessive credit card or other fraudulent activity, we could incur substantial costs.

        Most of our customers authorize us to bill their credit card or other online accounts directly for subscription fees that we charge. If people pay for our services with stolen credit cards or other wrongly obtained online account information, we could incur substantial third-party vendor costs for which we may not be reimbursed. Further, our customers provide us with credit card billing information online, and we do not review the physical credit cards used in these transactions, which increases our risk of exposure to fraudulent activity. We also incur charges, which we refer to as chargebacks, from the credit card and other online payment processing companies from claims that the customer did not authorize the credit card or other online transaction to purchase our subscription. If the number of unauthorized credit card or other online transactions becomes excessive, we could be assessed substantial fines for excess chargebacks and we could lose the right to accept credit cards or other online accounts for payment, which could have a material adverse effect on our business and operating results.

Our reported financial results may be adversely affected by changes in accounting principles generally accepted in the United States.

        Generally accepted accounting principles in the United States, or GAAP, are subject to interpretation by the Financial Accounting Standards Board, or the FASB, the Securities and Exchange Commission, or the SEC, and various bodies formed to promulgate and interpret appropriate accounting principles. A change in these principles or interpretations could have a significant effect on our reported financial results for periods prior to and subsequent to such change, and could affect the reporting of transactions completed before the announcement of a change.

        In particular, in May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition . The core principle of ASU 2014-09 is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services; this new accounting standard also impacts the recognition of sales commissions. As an "emerging growth company" the JOBS Act allows us to delay adoption of new or revised accounting pronouncements applicable to public companies until such pronouncements are made applicable to private companies. We have elected to use this extended transition period under the JOBS Act with respect to ASU 2014-09, and as a result ASU 2014-09 will become applicable to us on January 1, 2019.

        ASU 2014-09 permits the use of either a retrospective or cumulative effect transition method. We continue to evaluate the impact of the new standard and available adoption methods on our consolidated financial statements. We are in the process of evaluating arrangements with customers and identifying differences in accounting between new and existing standards. Regardless of the transition method, the application of this new guidance could have an adverse effect on our operating results in one or more periods as compared to what they would have been under current standards.

If our estimates or judgments relating to our critical accounting policies prove to be incorrect, our results of operations could be adversely affected.

        The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable under the circumstances, as provided in the section titled "Management's Discussion and Analysis of Financial Condition and Results of Operations." The results of these estimates form the basis for making judgments about the carrying values of assets, liabilities and equity, and the amount of revenue and expenses that are not readily apparent from other sources. Significant assumptions and estimates used in preparing our consolidated financial statements include those related to revenue

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recognition, valuation of goodwill and intangible assets, and stock-based compensation. Our results of operations may be adversely affected if our assumptions change or if actual circumstances differ from those in our assumptions, which could cause our results of operations to fall below the expectations of securities analysts and investors, resulting in a decline in the trading price of our common stock.

The requirements of being a public company may strain our resources, divert management's attention and affect our ability to attract and retain executive management and qualified board members.

        As a public company, we will be subject to the reporting requirements of the Securities Exchange Act of 1934, as amended, or the Exchange Act, the Sarbanes-Oxley Act of 2002, or the Sarbanes-Oxley Act, the Dodd-Frank Wall Street Reform and Consumer Protection Act, the listing requirements of the New York Stock Exchange and other applicable securities rules and regulations. Compliance with these rules and regulations will increase our legal and financial compliance costs, make some activities more difficult, time-consuming or costly, and increase demand on our systems and resources, particularly after we are no longer an "emerging growth company" as defined in the JOBS Act.

        In addition, changing laws, regulations and standards relating to corporate governance and public disclosure are creating uncertainty for public companies, further increasing legal and financial compliance costs, and making some activities more time consuming. These laws, regulations and standards are subject to varying interpretations, in many cases due to their lack of specificity, and, as a result, their application in practice may evolve over time as new guidance is provided by regulatory and governing bodies. This could result in continuing uncertainty regarding compliance matters and higher costs necessitated by ongoing revisions to disclosure and governance practices. We intend to invest substantial resources to comply with evolving laws, regulations and standards, and this investment may result in increased general and administrative expense and a diversion of management's time and attention from revenue-generating activities to compliance activities. If our efforts to comply with new laws, regulations and standards differ from the activities intended by regulatory or governing bodies due to ambiguities related to their application and practice, regulatory authorities may initiate legal proceedings against us and our business may be adversely affected.

        Moreover, the demands on management in operating a publicly-traded company, interacting with public company investors and complying with the increasingly complex laws pertaining to public companies, are significant. Our management team may not successfully or efficiently manage us as a public company subject to significant regulatory oversight and reporting obligations under the federal securities laws and the continuous scrutiny of securities analysts and investors. These new obligations and constituents require significant attention from our senior management and could divert their attention away from the day-to-day management of our business, which could adversely affect our business, results of operations and financial condition.

        We also expect that being a public company will make it more expensive for us to obtain director and officer liability insurance, and we may be required to accept reduced coverage or incur substantially higher costs to obtain coverage. These factors could also make it more difficult for us to attract and retain qualified members of our board of directors and qualified executive officers.

If we are unable to implement and maintain effective internal controls over financial reporting in the future, investors may lose confidence in the accuracy and completeness of our financial reports and the market price of our common stock may be negatively affected.

        As a public company, we will be required to maintain internal controls over financial reporting and to report any material weaknesses in such internal controls. Section 404 of the Sarbanes-Oxley Act requires that we furnish a report by management on, among other things, the effectiveness of our internal control over financial reporting for the year ending December 31, 2018. This assessment will need to include disclosure of any material weaknesses identified by our management in our internal control over financial

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reporting, as well as a statement that our independent registered public accounting firm has issued an opinion on our internal control over financial reporting, provided that our independent registered public accounting firm will not be required to attest to the effectiveness of our internal control over financial reporting until our first annual report required to be filed with the SEC following the date we are no longer an "emerging growth company," as defined in the JOBS Act.

        If we have a material weakness in our internal controls over financial reporting, we may not detect errors on a timely basis and our financial statements may be materially misstated. We are in the process of designing and implementing the internal controls over financial reporting required to comply with this obligation, which process will be time-consuming, costly and complicated. If we identify material weaknesses in our internal controls over financial reporting, if we are unable to comply with the requirements of Section 404 in a timely manner, if we are unable to assert that our internal controls over financial reporting are effective, or if our independent registered public accounting firm is unable to express an opinion as to the effectiveness of our internal controls over financial reporting, when required, investors may lose confidence in the accuracy and completeness of our financial reports, and the market price of our common stock could be negatively affected. In addition, we could become subject to investigations by the New York Stock Exchange, the SEC or other regulatory authorities, which could require additional financial and management resources and adversely affect our business, financial condition and operating results.

Risks Related to This Offering and Ownership of Our Common Stock

There has been no prior market for our common stock. An active market may not develop or be sustained and investors may not be able to resell their shares at or above the initial public offering price.

        There has been no public market for our common stock prior to this offering. The initial public offering price for our common stock will be determined through negotiations between the underwriters and us and may vary from the market price of our common stock following this offering. If you purchase shares of our common stock in this offering, you may not be able to resell those shares at or above the initial public offering price, if at all.

        Our common stock has been approved for listing on the New York Stock Exchange under the symbol "SEND." However, there can be no assurance that an active trading market for our common stock will develop on that exchange or elsewhere or, if developed, that any market will be sustained. The lack of an active trading market for our common stock could depress the market price of our common stock and could affect your ability to sell your shares, thus causing the value of any investment in our common stock to decline.

Our stock price may be volatile, and the value of any investment in our common stock may decline.

        The trading price of our common stock following this offering is likely to be highly volatile and could be subject to wide fluctuations in response to various factors, many of which are beyond our control. In addition to the factors discussed in this "Risk Factors" section and elsewhere in this prospectus, these factors include:

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our operating performance and the operating performance of similar companies;



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failure to meet revenue, earnings, key metrics or other financial or operational expectations that we establish or are established by securities analysts or investors;



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the overall performance and volatility of the equity markets in general or of our industry in particular;



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actual or anticipated developments in our business, our competitors' businesses or the competitive landscape generally;

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the number of shares of our common stock publicly owned and available for trading;



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threatened or actual litigation;



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changes in laws or regulations affecting our business;



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changes in our board of directors or management;



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publication of research reports about us or our industry or changes in recommendations or withdrawal of research coverage by securities analysts;



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public reaction to our press releases, other public announcements and filings with the SEC, including related to new services or functionalities or announced or completed acquisitions;



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changes in accounting standards, policies, guidelines, interpretations or principles;



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large volumes of sales of shares of our common stock by us or our existing stockholders; and



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general political and economic conditions.

        In addition, the stock market in general, and the market for technology companies in particular, has experienced extreme price and volume fluctuations that have often been unrelated or disproportionate to the operating performance of those companies. Broad market and industry factors may seriously affect the market price of our stock regardless of actual operating performance. These fluctuations may be even more pronounced in the trading market for our stock shortly following this offering. Securities class action litigation has often been instituted against companies following periods of volatility in the market price of their securities. This form of litigation, if instituted against us, could result in very substantial costs, divert our management's attention and resources and harm our business, operating results and financial condition.

As a new investor, you will experience immediate and substantial dilution in the book value of the shares that you purchase in this offering.

        The initial public offering price is substantially higher than the pro forma net tangible book value per share of our common stock immediately following this offering based on the total value of our tangible assets less our total liabilities. Therefore, if you purchase shares of our common stock in this offering, at the assumed initial public offering price of $14.50 per share, which is the midpoint of the estimated offering price range set forth on the cover page of this prospectus, you will experience immediate dilution of $11.05 per share, the difference between the price per share you pay for our common stock and our pro forma net tangible book value per share as of September 30, 2017, after giving effect to the issuance of 7,700,000 shares of our common stock in this offering. See "Dilution." To the extent outstanding options or warrants to purchase our common stock are exercised, investors purchasing our common stock in this offering will experience further dilution.

Substantial future sales of shares of our common stock could cause the market price of our common stock to decline.

        The market price of our common stock could decline as a result of substantial sales of our common stock, particularly sales by our directors, executive officers and significant stockholders, a large number of shares of our common stock becoming available for sale or the perception in the market that holders of a large number of shares intend to sell their shares. After this offering, there will be 40,291,661 shares of our common stock outstanding, based on the number of shares outstanding as of September 30, 2017, including the shares to be sold in this offering, which may be resold in the public market immediately. Substantially all of the remaining shares are currently restricted as a result of market stand-off agreements restricting their sale during the period ending 180 days after the date of this prospectus. In addition, substantially all of these shares are also subject to lock-up agreements with the representatives of the underwriters. Morgan Stanley & Co. LLC on behalf of the underwriters may, in its sole discretion, permit our officers, directors,

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employees and current security holders who are subject to lock-up agreements to sell shares prior to the expiration of the lock-up agreements.

        Additionally, 12,204,455 shares of our common stock subject to outstanding options and awards under our equity incentive plans as of September 30, 2017, will become eligible for sale in the public market in the future, subject to applicable vesting requirements, the lock-up agreements and market standoff provisions described above and Rules 144 and 701 under the Securities Act. See "Shares Eligible for Future Sale" for a more detailed description of sales that may occur in the future.

        We also intend to register all shares of our common stock that will be reserved for issuance under our 2017 Equity Incentive Plan and our 2017 ESPP. Once we register these shares, they can be freely sold in the public market upon issuance and once vested and exercised, as applicable, subject to the lock-up agreements described above.

        As of September 30, 2017, the holders of an aggregate of 24,471,780 shares of our convertible preferred stock, which shares of convertible preferred stock will be converted into 24,471,780 shares of our common stock upon the closing of this offering, have rights, subject to certain conditions, to require us to file registration statements covering their shares and to include their shares in registration statements that we may file for ourselves or other stockholders. The holder of the warrant to purchase 54,269 shares of our convertible preferred stock, which warrant will convert into a warrant to purchase 54,269 shares of our common stock upon the closing of this offering, also has these registration rights. The stockholders with these registration rights have waived such rights with respect to this offering.

        We may issue shares of our common stock or securities convertible into our common stock from time to time in connection with a financing, acquisition, investment or otherwise. Any such issuance could result in substantial dilution to our existing stockholders and cause the trading price of our common stock to decline.

Our management will have broad discretion over the use of the proceeds we receive in this offering and might not apply the proceeds in ways that increase the value of your investment.

        Our management generally will have broad discretion to use the net proceeds to us from this offering, and you will be relying on the judgment of our management regarding the application of these proceeds. Our management might not apply the net proceeds from this offering in ways that increase the value of your investment. We intend to use the net proceeds to us from this offering for working capital and other general corporate purposes, including developing and enhancing our technical infrastructure, platform and services, expanding our research and development efforts and selling and marketing operations, meeting the increased compliance requirements associated with our transition to and operation as a public company, and expanding into new markets. We may also use a portion to make acquisitions or investments. However, we do not have any agreements or commitments for any acquisitions or strategic investments at this time. Until we use the net proceeds to us from this offering, we plan to invest them, and these investments may not yield a favorable rate of return. If we do not invest or apply the net proceeds from this offering in ways that enhance stockholder value, we may fail to achieve expected financial results, which could cause our stock price to decline.

Concentration of ownership among our officers, directors, significant stockholders and their affiliates may prevent new investors, including purchasers in this offering, from influencing corporate decisions.

        Our officers, directors and their affiliated funds and each of our stockholders who own greater than 5% of our outstanding common stock and their affiliates, in the aggregate, will beneficially own an aggregate of approximately 66.5% of the outstanding shares of our common stock following this offering, assuming no exercise of the underwriters' option to purchase additional shares. If the existing stockholders associated with Bessemer Venture Partners, which is an affiliate of a member of our board of directors, purchase shares of our common stock with an aggregate price of $10.0 million in this offering (the

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maximum indicated interest, as described above), assuming no exercise of the underwriters' option to purchase additional shares, our officers, directors and their affiliated funds and each of our stockholders who own greater than 5% of our outstanding common stock and their affiliates, in the aggregate, will beneficially own an aggregate of approximately 68.1% of our outstanding common stock following this offering, assuming an initial public offering price of $14.50 per share, which is the midpoint of the estimated offering price range set forth on the cover page of this prospectus. As a result, if some of these persons or entities act together, they will have significant influence over the outcome of matters submitted to our stockholders for approval, including the election of directors and approval of significant corporate transactions, such as a merger or other sale of our company or our assets. This concentration of ownership could limit the ability of other stockholders to influence corporate matters and may have the effect of delaying or preventing a change of control of our company. Some of these persons or entities may have interests different from yours. For example, because many of these stockholders purchased their shares at prices substantially below the price at which shares are being sold in this offering and have held their shares for a relatively longer period, they may be more interested in selling the company to an acquirer than other investors.

We do not intend to pay dividends for the foreseeable future.

        We never have declared or paid any cash dividends on our capital stock and do not intend to pay any cash dividends in the foreseeable future. In addition, our credit facility restricts, and any future debt or preferred security arrangements may restrict, our ability to make distributions to our stockholders. We anticipate that we will retain any future earnings for use in the development of our business and for general corporate purposes. Any determination to pay dividends in the future will be at the discretion of our board of directors. Accordingly, investors must rely on sales of their shares of our common stock after price appreciation, which may never occur, as the only way to realize any future gains on their investments.

We are an "emerging growth company," and we cannot be certain if the reduced disclosure requirements applicable to emerging growth companies will make our common stock less attractive to investors.

        For so long as we remain an "emerging growth company," as defined in the JOBS Act, we may take advantage of certain exemptions from various requirements that are applicable to public companies that are not "emerging growth companies," including not being required to comply with the independent auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act, reduced disclosure obligations regarding executive compensation in our periodic reports and proxy statements, and exemptions from the requirements of holding a nonbinding advisory vote on executive compensation and stockholder approval of any golden parachute payments not previously approved. Further, we have elected to take advantage of the extended transition period to comply with new or revised accounting standards. As a result of the accounting standards election, we will not be subject to the same implementation timing for new or revised accounting standards as other public companies that are not emerging growth companies, which may make comparison of our financials to those of other public companies more difficult. We will remain an "emerging growth company" until the earliest of (i) the last day of the year following the fifth anniversary of the completion of our initial public offering, (ii) the last day of the first year in which our annual gross revenue is $1.07 billion or more, (iii) the date on which we have, during the previous rolling three-year period, issued more than $1.00 billion in non-convertible debt securities or (iv) the date on which we are deemed to be a "large accelerated filer" as defined in the Exchange Act. We cannot predict if investors will find our common stock less attractive because we may rely on these exemptions. If some investors find our common stock less attractive as a result, there may be a less active trading market for our common stock, and our stock price may be more volatile and may decline

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Anti-takeover provisions in our charter documents and under Delaware law could make an acquisition of our company more difficult, limit attempts by our stockholders to replace or remove our current management and limit the market price of our common stock.

        Provisions in our amended and restated certificate of incorporation and amended and restated bylaws to be effective in connection with the closing of this offering, may have the effect of delaying or preventing a change of control or changes in our management. Our amended and restated certificate of incorporation and amended and restated bylaws include provisions that:

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authorize our board of directors to issue, without further action by the stockholders, shares of undesignated preferred stock with terms, rights, and preferences determined by our board of directors that may be senior to our common stock;



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require that any action to be taken by our stockholders be effected at a duly called annual or special meeting and not by written consent;



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specify that special meetings of our stockholders can be called only by our board of directors, the chairperson of our board of directors, or our chief executive officer;



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establish an advance notice procedure for stockholder proposals to be brought before an annual meeting, including proposed nominations of persons for election to our board of directors;



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establish that our board of directors is divided into three classes, with each class serving three-year staggered terms;



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prohibit cumulative voting in the election of directors;



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provide that our directors may be removed for cause only upon the vote of sixty-six and two-thirds percent (66 ² / 3 %) of our outstanding shares of common stock;



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provide that vacancies on our board of directors may be filled only by a majority of directors then in office, even though less than a quorum; and



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require the approval of our board of directors or the holders of at least sixty-six and two-thirds percent (66 ² / 3 %) of our outstanding shares of common stock to amend our bylaws and certain provisions of our certificate of incorporation.

        These provisions may frustrate or prevent any attempts by our stockholders to replace or remove our current management by making it more difficult for stockholders to replace members of our board of directors, which is responsible for appointing the members of our management. In addition, because we are incorporated in Delaware, we are governed by the provisions of Section 203 of the Delaware General Corporation Law, which generally prohibits a Delaware corporation from engaging in any of a broad range of business combinations with any "interested" stockholder for a period of three years following the date on which the stockholder became an "interested" stockholder. Any delay or prevention of a change of control transaction or changes in our management could cause the market price of our common stock to decline.

Our amended and restated certificate of incorporation to be effective in connection with the closing of this offering will provide that the Court of Chancery of the State of Delaware or the U.S. federal district courts will be the exclusive forums for substantially all disputes between us and our stockholders, which could limit our stockholders' ability to obtain a favorable judicial forum for disputes with us or our directors, officers or other employees.

        Our amended and restated certificate of incorporation to be effective in connection with the closing of this offering provides that the Court of Chancery of the State of Delaware is the sole and exclusive forum for any derivative action or proceeding brought on our behalf, any action asserting a breach of fiduciary duty owed by any of our directors, officers or other employees to us or our stockholders, any action asserting a claim against us arising pursuant to any provisions of the Delaware General Corporation Law,

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our amended and restated certificate of incorporation or our amended and restated bylaws, or any action asserting a claim against us that is governed by the internal affairs doctrine. Our amended and restated certificate of incorporation further provides that the U.S. federal district courts will be the exclusive forum for resolving any complaint asserting a cause of action arising under the Securities Act. These choice of forum provisions may limit a stockholder's ability to bring a claim in a judicial forum that it finds favorable for disputes with us or our directors, officers or other employees, which may discourage such lawsuits. If a court were to find either choice of forum provision contained in our amended and restated certificate of incorporation to be inapplicable or unenforceable in an action, we may incur additional costs associated with resolving such action in other jurisdictions, which could adversely affect our results of operations and financial condition.

If securities or industry analysts do not publish research or publish inaccurate or unfavorable research about our business, our stock price and trading volume could decline.

        The trading market for our common stock will depend in part on the research and reports that securities or industry analysts publish about us or our business. If too few securities analysts commence coverage of us, the trading price for our common stock would be negatively affected. If one or more of the analysts who cover us downgrade our common stock or publish inaccurate or unfavorable research about our business, our common stock price would likely decline. If one or more of these analysts cease coverage of us or fail to publish reports on us regularly, demand for our common stock could decrease, which might cause our common stock price and trading volume to decline.

Participation in this offering by certain of our existing stockholders would reduce the public float for our shares.

        Certain of our existing stockholders associated with Bessemer Venture Partners, which is an affiliate of a member of our board of directors, have indicated an interest in purchasing shares of common stock with an aggregate price of up to $10.0 million in this offering. Because these indications of interest are not binding agreements or commitments to purchase, the underwriters may elect to sell fewer or no shares in this offering to such stockholders, or these stockholders may elect to purchase fewer or no shares in this offering. Assuming an initial public offering price of $14.50 per share, which is the midpoint of the estimated offering price range set forth on the cover page of this prospectus, if such stockholders were to purchase the entire $10.0 million of our common stock, they would purchase an aggregate of 689,655 shares of our common stock in this offering. If such stockholders purchase all such shares, assuming no exercise by the underwriters of their option to purchase additional shares, our officers, directors, and stockholders holding more than 5% of our capital stock as of September 30, 2017, and their affiliates, will beneficially own, in the aggregate, approximately 68.1% of our outstanding shares of common stock following this offering.

        If these stockholders are allocated all or a portion of the shares in which they have indicated an interest in this offering and purchase any such shares, such purchase would reduce the available public float for our shares because such stockholders would be restricted from selling the shares by a lock-up agreement they have entered into with our underwriters (subject to certain exceptions) and by restrictions under applicable securities laws. As a result, any purchase of shares by such stockholders in this offering may reduce the liquidity of our common stock relative to what it would have been had these shares been purchased by investors that were not existing stockholders.

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