Messages from a particular source or domain are accepted by Messaging Gateway (SMG) but a specific message always fails to be delivered with a message indicating that the message delivery timed out.
421 4.4.2 service timed out
When processing a specific message, the SMG mail service does not respond to the end of the message resulting in an SMTP session time out.
This is a known issue with a very specific and limited message format and does not generally affect Messaging Gateway's ability to accept and deliver SMTP email.
This issue will be addressed with a future release.