If you receive this Pop Up
1) Open Outlook 2016
2) Click the Mimecast tab on your Outlook Ribbon
3) Once the Mimecast tab is open, click Account Settings
4) Once Account Settings is open, click Fix
5) Once you have clicked Fix, enter your Windows Password in the field below your Email Address, afterward, click Submit Password
If you have entered the correct credentials, your Domain Aunthentication will become Active. You will be able to use Mimecast and its features at this point.