The "unknown mail transport error" in Postfix, resulting in suspended delivery, generally indicates that a transport defined in main.cf is missing from master.cf or that a relayhost has invalid syntax. Troubleshooting involves validating master.cf service definitions, checking relayhost brackets, and rebuilding transport maps. Read more at Server Fault .
Useful commands
In short: Postfix tried to deliver, something external failed in a non-standard way, and Postfix is punting the problem to the queue. The "unknown mail transport error" in Postfix, resulting
"Delivery temporarily suspended: unknown mail transport error" is a frustratingly vague Postfix error message that can bring email communication to a halt. When this occurs, it indicates that the Postfix mail transfer agent (MTA) is attempting to send a message but cannot identify the destination transport mechanism, often leading to queued mail and deferred delivery. This article provides a comprehensive guide to understanding, diagnosing, and resolving this specific Postfix error. Understanding the "Unknown Mail Transport Error" in Postfix Useful commands In short: Postfix tried to deliver,
: If you are using a third-party milter or content filter (like Postgrey or Amavis ), and that service is crashed or not listening on the expected socket/port, Postfix will suspend delivery to that path. Troubleshooting Steps resulting in suspended delivery