sudo tail -100 /var/log/mail.log # or on RHEL/CentOS: /var/log/maillog Look for lines surrounding the error. A typical failure block might look like this:
This article provides a deep dive into the root causes of this error, specifically focusing on scenarios that arise . We will explore why this happens, how to diagnose the exact issue, and step-by-step solutions to restore normal mail flow. Understanding the Error: What Does It Mean? In Postfix terminology, a transport is the method by which a message is delivered to its final destination. This could be a local mailbox (e.g., using local transport), an LMTP socket, a Dovecot deliver agent, or a relay host (e.g., smtp transport). sudo tail -100 /var/log/mail
sudo journalctl -u dovecot --since "10 minutes ago" sudo journalctl -u spamassassin Scenario 1: Broken Dovecot LMTP or deliver binary Symptom: You updated Postfix but also updated Dovecot from version 2.2 to 2.3 (or similar). The error appears for local or virtual deliveries. Understanding the Error: What Does It Mean
Procmail (or the custom binary) was removed or replaced by a stub that returns a non-zero exit code. Many distributions removed Procmail by default after 2020. sudo journalctl -u dovecot --since "10 minutes ago"
dovecot unix - n n - - pipe To (add v flag and increase verbosity):
Dovecot changed the location of its binary or the protocol for LMTP sockets. For example, dovecot-lda (Local Delivery Agent) may have moved from /usr/lib/dovecot/deliver to /usr/libexec/dovecot/dovecot-lda .