Mimecast Outlook add-on

This guide should allow you to re-enable the mimecast add-on in outlook

Click the File menu
Select Options
Select the Add-Ins section
Select COM Add-ins from the Manage drop-down list and click Go...
In the COM Add-ins dialog box, ensure that the Mimecast Services for Outlook add-in is enabled by checking the appropriate box.
Ensure that Mimecast Services for Outlook is listed under the Active Application Add-ins section, and not under the Disabled Application Add-ins section.
Click the OK button to close the window, and click OK to return to Outlook.