Messages for a specific domain or email address are not being delivered as expected. Reviewing the Message Audit Logs for one or more failed messages shows the following SMTP error code:
554 5.4.6 Hop count exceeded - possible mail loop
This SMTP response indicates that there is a mail loop in the environment. Messaging Gateway is routing abd deliverying the domain or email address as configured but the message is then being routed and delivered back to Messaging Gateway. As the message traverses this routing loop, it accumulates Received headers until one of the email servers in the loop detects the mail loop and refuses delivery with an SMTP code that causes the undeliverable message to be deleted.