MIME 2018 Proxy Statement

July 27, 2018 Dear Mimecast Shareholder: I am pleased to invite you to attend the 2018 Annual General Meeting (the “Annual Meeting”) of Mimecast Limited (“Mimecast” or the “Company”) to be held on Thursday, October 4, 2018, at 10:00 a.m. (London time), at Mimecast’s offices located at City Point, One Ropemaker Street, Moorgate, London EC2Y 9AW, United Kingdom. Details regarding the Annual Meeting and the business to be conducted are more fully described in the accompanying Notice of 2018 Annual General Meeting of the Company and Proxy Statement. We have also enclosed our Annual Report on Form 10-K for the year ended March 31, 2018, which contains, among other things, our audited consolidated financial statements for the year ended March 31, 2018. Your vote is important. Whether or not you plan to attend the Annual Meeting, I hope you will vote as soon as possible. You may vote by completing your proxy card and mailing it, by telephone or in person at the Annual Meeting. Please review the instructions on the proxy card regarding your voting options. Thank you for being a Mimecast shareholder. We look forward to seeing you at our Annual Meeting. Sincerely, Peter Bauer Chief Executive Officer YOUR VOTE IS IMPORTANT In order to ensure your representation at the Annual Meeting, whether or not you plan to attend the meeting, please vote your ordinary shares as promptly as possible by following the instructions on your proxy card. Your participation will help to ensure the presence of a quorum at the meeting and save Mimecast the extra expense associated with additional solicitation. If you hold your ordinary shares through a broker, your broker is not permitted to vote on your behalf on (i) the election of directors, (ii) the non-binding, advisory vote to receive the Company’s accounts for the year ended March 31, 2018, together with the directors’ report and the auditor’s report on those accounts, (iii) the non-binding, advisory vote to approve the compensation of our named executive officers, or (iv) the non-binding, advisory vote on the frequency of future advisory votes to approve the compensation of our named executive officers, unless you provide specific instructions to the broker by completing and returning any voting instruction form that the broker provides (or following any instructions that allow you to vote your broker-held ordinary shares via telephone or the Internet). For your vote to be counted, you will need to communicate your voting decision before the date of the Annual Meeting. Voting your ordinary shares in advance will not prevent you from attending the Annual Meeting, revoking your earlier submitted proxy or voting your ordinary shares in person.