: If main.cf is configured to use a specific transport (e.g., via relayhost , transport_maps , or content_filter ) that is not explicitly defined or is commented out in master.cf , Postfix will report an unknown transport error.
This status is a "soft failure" or deferral, meaning Postfix will keep the mail in the queue and try again later, but it signal that a underlying configuration or service issue is blocking the path. Primary Causes of the "Unknown Mail Transport Error" : If main
: Minor spelling errors in service names within master.cf can cause the qmgr to fail its connection to the necessary socket. Common examples include typos in custom filter names like amavis or spamassassin . Common examples include typos in custom filter names
To resolve this issue, you must identify the specific transport that is failing and why it is unavailable. Error unknown mail transport error - Virtualmin Community Troubleshooting Steps
: 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