Click the File tab.

Click on the Options menu item.

Click on the Add-Ins menu item. The View and Manager Microsoft Outlook Add-Ins dialog is displayed.

Select COM Add-ins in the "Manage" drop down list at the bottom of the dialog.

Click the Go... button. The COM Add Ins dialog is displayed.

Ensure the Mimecast for Outlook add-in is enabled. Clicking the tick box to the left of it if required.

Click on the OK button to close the window and return to Microsoft Outlook.