Recently had a customer stop being able to send items to Mimecast with the following 

554 Email rejected due to security policies - https://community.mimecast.com/docs/DOC-1369#554

This was happening to Multiple emails and Mimecast Logs showed : Bounce – Message content not accepted

Also users trying to email the customer by replying to their email would get the same bounce back

So something generic in the content was causing this

Generic would be email signatures and Links in email signatures are usually the cause, removing one of the domains in the email signature fixed this , we lodged a Support case with Mimecast who removed this domain from their blocklist

The actual cause was the company was sending mass emails with using something like mailchimp so it did not have an unsubscribe button so domains in the email were blocked in third party lists like : http://lookup.uribl.com/