The configuration file /etc/postfix/main.cf contains the essential directives for your mail server. An incorrect or malformed entry here can immediately break mail delivery. Common mistakes include:
The message "unknown mail transport error" is simply a generic catch-all warning. The descriptive error indicating exactly why the transport failed is located earlier in your mail log. Open your terminal and isolate the transaction logs using grep with your system's specific log file: The configuration file /etc/postfix/main
What (e.g., Dovecot, Amavis, OpenDKIM) you are running. The descriptive error indicating exactly why the transport
Did you run postmap after changing transport files? The error message alone is insufficient
The error message alone is insufficient. You need to correlate it with other log entries. Run the following command to view recent mail logs:
The critical piece here is the QUEUE_ID . You'll use this to cross-reference with the logs.
After making corrections to your configuration files, apply the changes and force Postfix to retry sending the stuck messages. sudo postfix check Use code with caution. If this returns nothing, your syntax is clean. Restart the Postfix service: sudo systemctl restart postfix Use code with caution.