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 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 This status is a "soft failure" or deferral,
: Postfix requires strict ownership (usually root or postfix ) and permissions for its spool directories, sockets, and configuration files. Incorrect permissions can prevent the queue manager from communicating with the delivery agents. Incorrect permissions can prevent the queue manager from
Most instances of this error stem from misalignments between the two main Postfix configuration files: main.cf and master.cf . and configuration files.