Problem:
if userA@mydomain.com sends an Email to userB@mydomain.com ... they often wind up in quarantine on Proofpoint Essentials.
Root Cause:
Unfortunately if your mail is hosted on Rackspace, contrary to on-prem Microsoft Exchange or Office365 where two users communicating with one another never leaves the premise, inter-user emails controlled by Rackspace follow MX resolution so the emails going from UserA to UserB transit through proofpoint instead of staying internal to rackspace.
Workaround:
You need to create a filter rule that basically says:
if any emails for *@mydomain.com come in and they have rackspace header elements in them, then allow them.
Example:
