1. If you receive the Mimecast for Outlook Authentication required error pop-up box (screenshot below), click where it says “Click here to authenticate your account”
If you had already closed the above window before clicking on it, you can also access by going to the Mimecast tab in your Outlook Client, and clicking Account Settings
2. Once you receive the below screen, click the Authenticate button
3. Then you will receive the following window and you will enter your network password and click Submit Password.