Delivery Temporarily Suspended Unknown Mail Transport Error Postfix Upd Jun 2026

Scenario B: Permission or Configuration Issues with Dovecot (LMTP/LDA)

In Postfix, a "transport" tells the mail system how to deliver a message (e.g., via local delivery, SMTP over the internet, or a custom script). If Postfix receives a message but cannot determine the correct transport method, or if the transport it tries to use is broken, it triggers this error. Key Causes and Solutions 1. Corrupt or Misconfigured master.cf File (Most Common)

postsuper -r ALL deferred

If the transport is a script, try running it manually as the user defined in master.cf .

smtp-source -v -p 25 example.com

Incorrect entries in transport maps, main.cf, or master.cf often cause this error. For example, an invalid or incomplete transport map can leave Postfix unable to route a message. Similarly, specifying a relayhost incorrectly can cause a lookup failure that manifests as this generic transport error. Postfix is highly sensitive to the exact syntax of its configuration directives.

Following a system or Postfix update, your lookup tables (e.g., sasl_passwd ) might need to be re-indexed using the newaliases Virtualmin Community How to Troubleshoot Check the Real Error Logs

Look for lines containing panic , fatal , or warning right next to the affected Message-ID. For example, if you see fatal: transport amavis swallows data , it means Postfix is looking for a service named "amavis" that is broken or missing. 2. Verify Your Transport Maps

Run sudo postfix set-permissions to fix any internal ownership issues. Scenario B: Permission or Configuration Issues with Dovecot

Your first and most important action is to look beyond the initial error and find the preceding log messages. Postfix almost always logs a specific reason for the failure just before the generic "unknown mail transport error".

Look at the full error log, specifically the part after "unknown mail transport error:". This usually tells you which service ( smtp , local , uucp , custom_service ) is failing.

Look 10 lines above the "unknown mail transport error." The true error (e.g., “Name service error,” “Connection timed out,” or “TLS read error”) lives there.

The error message in Postfix typically indicates that the mail queue manager ( qmgr ) has attempted to hand off a message to a specific delivery transport (like smtp , local , or a custom filter) but encountered a critical failure that prevents any further attempts for that destination for a period. Corrupt or Misconfigured master

Once you've identified the root cause, you can apply the appropriate fix.

If any partition (especially / or /var ) is at 100%, delete old logs, clear package caches, or expand the storage volume. Scenario D: Broken Chroot Environment

df -h # Checks general disk space df -i # Checks inode usage Use code with caution.