Messages to unreachable domains are bounced
Symptoms: Outbound messages are bounced with one of the following errors:
"550 MTA inaccessible for too long for domain: - psmtp"
"550 MX records inaccessible for too long for domain: - psmtp"
These messages may have previously been stuck in the sending mail server's queue with the following 400-series deferral:
"451 Cannot connect to domain: - psmtp"
Resolution: The message security service returns a 451 deferral when the receiving domain is unreachable. This is because either the attempt to look up the domain's MX records failed, or none of the servers listed in the domain's MX records are reachable.
The message security service continues to return the 451 deferral on each retry for up to six hours. After six hours, the messages are bounced with the appropriate error above to remove the message from the queue. This is done to keep queue length to a minimum and to inform the sender within a reasonable amount of time that the message was not successfully delivered.
The outbound message security service aggregates information from all senders. This means that you may or may not experience the six-hour deferral window described above before the messages are bounced. If other outbound customers have been attempting to send to the same unreachable domain, the service keeps track of it and begins bouncing the messages six hours after the first unsuccessful message was sent, regardless of the originating customer.
Note: This article is intended for administrators using Message Filtering, Message Security, Message Discovery, and Message Encryption. If you're using another edition, your service may include different features from those described in this article.
Keywords: outbound, deferral, deferred, bounced, failed, returned, 550, MTA inaccessible for too long, MX records inaccessible for too long, 451 Cannot connect to domain: