This error message in your Postfix logs (/var/log/mail.log or /var/log/maillog) is a source of frustration for many system administrators. It indicates that Postfix attempted to hand off an email to a specific transport mechanism (like a content filter, a relay host, or a custom script), but that hand-off failed unexpectedly.
Because the failure happened at the "transport" layer rather than the SMTP protocol layer, Postfix does not know exactly why it failed, so it marks the status as "unknown" and places the email in a deferred queue to try again later.
If the transport is amavis or similar:
First, take a deep breath. “Temporarily” is the key word. This is a soft error (a 4.x.x SMTP code), not a permanent failure (5.x.x). Postfix is telling you: “I tried to deliver this message, but something went wrong that might be fixed later. I’ll keep trying.”
The real problem is the phrase: “unknown mail transport error.” This is Postfix’s way of saying: “I know there is a transport problem, but I can’t pinpoint the exact SMTP error code from the remote server.” This error message in your Postfix logs ( /var/log/mail
The "unknown mail transport error" is a summary. The actual error usually appears 1–3 lines above it in the logs.
Run this command to view recent activity: Look for lines mentioning warning , error , or fatal
tail -f /var/log/mail.log
Look for lines mentioning warning, error, or fatal.