Open Outlook and click on the Mimecast tab in the ribbon. 

 

 

Click on Managed Senders

 

 

Under Managed Senders click on Permitted

 

 

Click on Add Permitted

 

 

Enter the email address or domain to be permitted and click add. 

 

 

If you have more that one address that you would like to add, enter it and click add again. Click Permit when finished 

 

 

The email(s) or domain(s) you added will now be listed. Close the window.