To (add v flag and increase verbosity):
dovecot unix - n n - - pipe
Postfix, the rock-solid Mail Transfer Agent (MTA) that underpins a vast swath of the internet’s email infrastructure, is usually a paragon of clarity. It either accepts a message, rejects it with a clear code (like the famous "550 User unknown"), or bounces it back. But the "unknown mail transport error" stands apart. It is the catch-all for the uncatchable—the digital equivalent of "something happened, but we’re not sure what." To (add v flag and increase verbosity): dovecot
The transport is a pipe or smtp to a local filter socket. You see "unknown mail transport error" along with status=exited or signal=SEGV in system logs. It is the catch-all for the uncatchable—the digital
: If using a database for virtual users, verify the service (e.g., systemctl status postgresql ) is active. verify the service (e.g.