Delivery Temporarily Suspended Unknown Mail Transport Error Postfix Upd «REAL»

If the relayhost parameter is set incorrectly or uses a bracketed format [host] that the system cannot resolve, it may trigger a transport error.

The key word in this error is temporarily . Unlike a fatal bounce (which sends a failure report to the sender), a temporary suspension tells Postfix to queue the message and try again later. This is typically governed by the minimal_backoff_time and maximal_backoff_time parameters. The phrase "unknown mail transport error" is Postfix’s default fallback when a lower-level component (like the local delivery agent local , the virtual agent virtual , or a pipe to a content filter) returns an exit code or signal that does not map to a specific known error (e.g., "permission denied" or "disk full"). If the relayhost parameter is set incorrectly or