Bad MX Record

As a general rule the ABI Hosting mail servers will not accept email if it can determined that the sending email address can not be responded to.  If a domain purposely publishes MX records which can not be reached by anyone on the internet (i.e., by using an RFC 1918 private IP address in the address record), the email will be rejected.

The solution to this is to be sure the MX records for the sending domain are properly configured.