The report is prepared for the general information of shareholders. It is authorized for distribution to prospective investors only when preceded or accompanied by a current prospectus.

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LETTER FROM THE EXECUTIVE CHAIR

Dear Shareholders:

Markets experienced dramatic swings in early 2020 as the coronavirus pandemic crippled the global economy for a time. Optimism over the development of vaccines

and therapeutics, along with a decline in cases in countries hit early in the outbreak, has brightened the economic and market outlook during the second quarter, though a great deal of uncertainty remains on how long the aftereffects of the lockdowns will linger.

Global central banks have taken aggressive, coordinated steps to cushion the economic and market fallout related to the virus, and governments are undertaking unprecedented levels of fiscal stimulus. As uncertainty recedes, these measures can help build a supportive environment and encourage economic recovery. In the aftermath of the crisis, there are likely to be societal changes as

households, businesses, and governments adjust to a new reality, and these alterations could change the investment landscape. For investors, occurrences such as the COVID-19 outbreak demonstrate the importance of having a deep understanding of company fundamentals, and our global research platform has been built to do just that.

Here at MFS®, we aim to help our clients navigate the growing complexity of the markets and world economies. Our long-term investment philosophy and commitment to the responsible allocation of capital allow us to tune out the noise and uncover what we believe are the best, most durable investment opportunities in the market. Through our powerful global investment platform, we combine collective expertise, thoughtful risk management, and long-term discipline to create sustainable value for investors.

Respectfully,

Robert J. Manning

Executive Chair

MFS Investment Management

June 16, 2020

The opinions expressed in this letter are subject to change and may not be relied upon for investment advice. No forecasts can be guaranteed.

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PORTFOLIO COMPOSITION

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Portfolio Composition – continued

Where the fund holds convertible bonds, they are treated as part of the equity portion of the portfolio.

Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statement of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Other includes equivalent exposure from currency derivatives and/or any offsets to derivative positions.

Percentages are based on net assets as of April 30, 2020.

The portfolio is actively managed and current holdings may be different.

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MANAGEMENT REVIEW

Summary of Results

For the twelve months ended April 30, 2020, Class A shares of the MFS Corporate Bond Fund (fund) provided a total return of 10.00%, at net asset value. This compares with a return of 9.37% for the fund’s benchmark, the Bloomberg Barclays U.S. Credit Bond Index.

Market Environment

Market volatility soared at the end of the period as COVID-19, the illness caused by the novel coronavirus first detected in China, spread around the world in early 2020. Government-imposed social distancing measures taken to slow the spread of the virus forced the shutdown of large portions of the global economy. In an attempt to limit the economic fallout of the crisis, central bankers and policymakers have taken a variety of actions unprecedented in both size and speed, but despite their best efforts, a global recession has begun. Given the unique circumstances surrounding the genesis of the downturn, it is difficult to forecast how long the crisis will linger.

High levels of investor uncertainty saw equity prices plunge, market volatility rise and large segments of global fixed income markets freeze as investors sought liquidity wherever they could find it. As the global economy rapidly slowed, a disagreement between Saudi Arabia and Russia over OPEC+ production cuts resulted in the Saudis shelving the OPEC+ arrangement and increasing oil output at the same time that global oil demand was plummeting, which compounded market turmoil. The resulting drop in prices undercut oil exporters, many of which are in emerging markets, as well as a large segment of the high-yield credit market.

Central banks around the world responded quickly and massively to the crisis with programs to improve liquidity and support markets. They proved largely successful in helping to restore market function, easing volatility and stimulating a market rebound at the end of the period. Monetary easing measures were complimented by large fiscal stimulus plans in developed countries. Even emerging market countries were able to implement counter-cyclical policies, a departure from the usual market-dictated response to risk-off crises, due to relatively manageable external liabilities and balance of payments in many countries and persistently low inflation.

As is often the case in a serious crisis, market vulnerabilities have been revealed. For example, companies that have added significant leverage to their balance sheets in recent years by borrowing to fund dividend payments and stock buybacks may find it difficult to follow that path in the future, and investors may not reward them if they do. Recapitalizations by some of these highly leveraged firms could dilute existing shareholders. Very long supply chains designed to squeeze out every bit of savings may be shifted closer to home markets if companies seek to emphasize resiliency over cost after a series of supply disruptions. The aftereffects of the global pandemic could impact consumer, business and government behavior in ways it will be difficult to forecast.

Factors Affecting Performance

Relative to the Bloomberg Barclays U.S. Credit Bond Index, the fund’s underweight allocation to both the energy and government owned, no guarantee sectors, as well as

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Management Review – continued

its overweight exposure to the electric sector, aided results. Favorable security selection within both the “A” and “BBB” rated ^(r) credit quality segments also contributed to relative performance.

Conversely, positioning along the yield curve ^(y) and the fund’s shorter duration ^(d) stance hurt relative performance over the reporting period. Bond selection within both the consumer cyclical and institutional finance sectors further weighed on relative returns.

Respectfully,

Portfolio Manager(s)

Alexander Mackey, Henry Peabody, and Robert Persons

Note to Shareholders: Effective December 31, 2019, Henry Peabody was added as a Portfolio Manager of the Fund.

The views expressed in this report are those of the portfolio manager(s) only through the end of the period of the report as stated on the cover and do not necessarily reflect the views of MFS or any other person in the MFS organization. These views are subject to change at any time based on market or other conditions, and MFS disclaims any responsibility to update such views. These views may not be relied upon as investment advice or an indication of trading intent on behalf of any MFS portfolio. References to specific securities are not recommendations of such securities, and may not be representative of any MFS portfolio’s current or future investments.

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PERFORMANCE SUMMARY THROUGH 4/30/20

The following chart illustrates a representative class of the fund’s historical performance in comparison to its benchmark(s). Performance results include the deduction of the maximum applicable sales charge and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. The performance of other share classes will be greater than or less than that of the class depicted below. Benchmarks are unmanaged and may not be invested in directly. Benchmark returns do not reflect sales charges, commissions or expenses. (See Notes to Performance Summary.)

Performance data shown represents past performance and is no guarantee of future results. Investment return and principal value fluctuate so your shares, when sold, may be worth more or less than the original cost; current performance may be lower or higher than quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on fund distributions or the redemption of fund shares.

Growth of a Hypothetical $10,000 Investment

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Performance Summary – continued

Total Returns through 4/30/20

Average annual without sales charge