I have had two instances where email is returned from MS Exchange with the following:
You do not have permission to send to this recipient. For assistance, contact your system administrator.
<servername.local #4.7.1 smtp;452 4.7.1 Spam filter message initialization failure - client - network: firstname.lastname@example.org>
If you restart CleanMail Server the problem seems to go away. Has anyone else seen this?