Check the version of Outlook you have on your PC (32-bit or 64-bit)> The version of Outlook you have may not be the same as your operating system.

Open Outlook > File > Office Account > About Outlook > Close Outlook

3. Go to https://community.mimecast.com/s/mimecast-for-outlook 
and then Select the version of your Outlook > Run the installer > Follow on screen instructions

4. Open Outlook > Go to Mimecast tab > Account settings > Select Domain Authentication > Fix

5. Enter your domain password