X0306 4 2021-04-26 0 0001644675 Mimecast Ltd MIME 0001789268 DiMarino Dino C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON MA 02421 0 1 0 0 Chief Revenue Officer Ordinary Shares 2021-04-26 4 M 0 2000 16.64 A 16156 D Ordinary Shares 2021-04-26 4 M 0 2000 34.82 A 18156 D Ordinary Shares 2021-04-26 4 S 0 4500 44.53 D 13656 D Restricted Share Units 1875 D Restricted Share Units 7500 D Restricted Share Units 11250 D Restricted Share Units 22469 D Restricted Share Units 29375 D Share Option (Right to Buy) 16.64 2021-04-26 4 M 0 2000 0.00 D 2026-09-01 Ordinary Shares 2000 6125 D Share Option (Right to Buy) 34.82 2021-04-26 4 M 0 2000 0.00 D 2028-04-02 Ordinary Shares 2000 23000 D The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on September 16, 2020. Each restricted share unit represents a contingent right to receive one share of the Issuer's Ordinary Shares. These restricted share units were granted on April 2, 2018. Twenty-five percent (25%) of the units vested on April 2, 2019, twenty-five percent (25%) of the units vested on April 2, 2020, and the remainder of the units vest on an annual basis over the next two (2) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. These restricted share units were granted on April 1, 2019. Twenty-five percent (25%) of the units vested on April 1, 2020 and the remainder of the units vest on an annual basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. These restricted share units were granted on October 1, 2019. Twenty-five percent (25%) of the units vested on October 1, 2020, and the remainder of the units vest on an annual basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. These restricted share units were granted on April 1, 2020. Twenty-five percent (25%) of the units vest on April 1, 2021 and the remainder of the units vest on an annual basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. These restricted share units were granted on April 1, 2021. Twenty-five percent (25%) of the units vest on April 1, 2022 and the remainder of the units vest on an annual basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. This option was granted on September 1, 2016. Twenty-five percent (25%) of the shares vested on September 1, 2017 and the remainder of the shares vested on a quarterly basis over the next three (3) years. This option was granted on April 2, 2018. Twenty-five percent (25%) of the shares vested on April 1, 2019, and the remainder of the shares vest on a quarterly basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. Exhibit 24 Power of Attorney (incorporated by reference to Power of Attorney filed as Exhibit 24 to Form 3 filed by the Reporting Person on October 1, 2019). /s/ Robert P. Nault, Attorney-in-Fact 2021-04-27