If your transport is configured to listen on a TCP port or Unix socket, and that port/socket is already taken by another process or has incorrect ownership permissions, Postfix cannot connect.
Locate the correct binary:
Seeing the "delivery temporarily suspended: unknown mail transport error" message in your Postfix mail logs ( /var/log/mail.log or via journalctl ) indicates a critical communication breakdown. Postfix wanted to pass a message to a specific delivery agent, but that agent is either missing, misconfigured, or crashing.
tail -f /var/log/mail.log (Look for "fatal: out of memory" or "limit exceeded" near the transport error). Summary Checklist DNS: Can the server see the internet?
Follow these steps in sequence to diagnose and resolve the transport suspension. 1. Analyze the Mail Logs If your transport is configured to listen on
[Current Date] System/Application: Postfix Mail Server
The "delivery temporarily suspended: unknown mail transport error" in Postfix is almost always a sign of a configuration issue, usually within master.cf or transport mappings. By carefully validating the configuration files and ensuring that custom transports are properly defined, you can usually resolve this issue quickly and get your mail flowing again.
Set up log parsers like Logwatch or integration daemons to scan your mail logs for the word suspended or deferred , alerting your team before users notice delivery delays.
: Ensure default_transport is not accidentally set to an error loop string, such as default_transport = error:No outside emails . The standard default parameter value is typically smtp . 3. Recompile Lookup Tables and Map Files tail -f /var/log/mail
Generate a custom policy module if needed (using audit2allow ).
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often sit between Postfix and the final destination. If these services crash or if the connection port (often 10024 or 10026) is blocked, the transport becomes unavailable. Chroot & Permission Issues
If this error is currently clogging your mail queue, this guide will help you diagnose the root cause and implement a permanent fix. Step 1: Locate and Inspect Your Mail Logs 3. Check Chroot and Permissions
Because it is a protective mechanism to prevent mail loops or data loss, Postfix pauses the queue. Here is a comprehensive guide to diagnosing and fixing this error. 1. Locate the Root Cause in the Logs
ls -l /usr/bin/spamfilter.sh
The receiving server is up, but it refused to accept the message at the TCP or TLS level. Causes include:
: If you changed map files (like sasl_passwd or transport ), run postmap /etc/postfix/ to update the .db files Postfix actually reads. 3. Check Chroot and Permissions