I have seen the classic you do not have permission to send to this recipient error before. However, I have never seen it followed up with a domain.com address not supported. Here is the entire NDR....
You do not have permission to send to this recipient. For assistance, contact your system administrator.
<mail.mydomain.org #5.7.1 smtp;550 5.7.1 <email@example.com>...destdomain,com address not supported>
Any ideas you can throw my way would be great.