1. Download the DMG installer from this link https://community.mimecast.com/community/knowledge-base/application-downloads/pages/mimecast-for-mac

2. Go to Downloads folder and run the installer > Drag the Mimecast App into Applications 

3. Open Mimecast in Applications > Enter your email address in the Log In page

4. Select "Domain" for Authentication Type > Enter your domain password

5. Now you can manage your emails with Mimecast. Under "Personal On Hold" you can check emails that have been held my Mimecast. You can release, permit, reject or block an email address using Mimecast when you right click on the email. You can also use "Quick Search" to search for emails just like when you search in Outlook.