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( Mark One ) ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended March 31 , 2021 OR UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM ΤΟ Commission File Number 001-37637 Bailiwick of Jersey ( State or other jurisdiction of incorporation or organization ) 1 Finsbury Avenue London EC2M 2PF United Kingdom ( Address of principal executive offices ) MIMECAST LIMITED ( Exact name of Registrant as specified in its Charter ) ( Title of each class ) Ordinary Shares , nominal value $ 0.012 per share Large accelerated filer Non - accelerated filer 冈 Registrant's telephone number , including area code : ( 781 ) 996-5340 Securities registered pursuant to Section 12 ( b ) of the Act : ( Trading Symbol ) MIME Securities registered Not Applicable ( I.R.S. Employer Identification No. ) EC2M 2PF ( Zip Code ) pursuant to Section 12 ( g ) of the Act : None ( Title of class ) ( Name of each exchange on which registered ) The Nasdaq Global Select Market Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . YES NO Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15 ( d ) of the Act . YES NO Indicate by check mark whether the registrant : ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . YES NO Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . YES > NO Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , smaller reporting company , or an emerging growth company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . ooo 0 Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C.7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . YES NO The aggregate market value of the voting and non - voting common equity held by non - affiliates of the registrant , based on the closing price of our ordinary shares on the Nasdaq Global Select Market on September 30 , 2020 , the last business day of the registrant's second fiscal quarter , was approximately $ 2,754 million . This calculation does not reflect a determination that certain persons or entities are affiliates of the registrant for any other purpose . The number of registrant's ordinary shares outstanding as of May 20 , 2021 was 65,169,227 . DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant's Definitive Proxy Statement relating to the 2021 Annual General Meeting of Shareholders of Mimecast Limited , expected to be held on October 6 , 2021 , are incorporated by reference into Part III of this Annual Report on Form 10 - K . The Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days of the registrant's fiscal year ended March 31 , 2021. Except with respect to information specifically incorporated by reference into this Annual Report on Form 10 - K , the Definitive Proxy Statement is not deemed to be filed as part of this Annual Report on Form 10 - K .