Here’s how to fix it:

1. Click on File - Manage COM Add-ins:

2. Select Options under Mimecast

3. Click on Enable.

If Mimecast doesnt appear in Manage COM Add-ins:

1. File - Options. Then select Add-ins

Click on the Manage dropdown and select: Disabled Items

Click on Go..

You should see the following window pop up:

Select Mimecast and click Enable.

Please then restart Outlook.