Edit /etc/postfix/master.cf and modify the transport line from:
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 . Edit /etc/postfix/master
The filter binary was recompiled but its dependencies (e.g., Perl modules, libssl) are now incompatible with the version Postfix is trying to run. Procmail (or the custom binary) was removed or
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. When you run a Postfix mail server—especially after
Introduction Few things are as frustrating for a mail server administrator as a vague error message. When you run a Postfix mail server—especially after a routine system update using apt update , yum update , or a manual source compilation—you might start seeing a cryptic message in your mail logs: "delivery temporarily suspended: unknown mail transport error" This message is a digital warning light. It tells you something is wrong, but it doesn’t tell you what. The word "unknown" is particularly alarming because it suggests Postfix itself cannot categorize the nature of the failure.
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.
grep -E "^dovecot" /etc/postfix/master.cf Output example: