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.
By methodically isolating the transport—whether Dovecot, Amavis, Maildrop, or a custom script—you can convert the "unknown" into a known, actionable fix. And once resolved, safeguard your configuration to ensure that the next system update doesn’t leave your mail queue suspended once again.
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). Procmail (or the custom binary) was removed or
grep -E "^dovecot" /etc/postfix/master.cf Output example:
dovecot unix - n n - - pipe flags=DRhu user=vmail argv=/usr/libexec/dovecot/dovecot-lda -f $sender -d $user@$domain -o plugin/quota=maildir:User quota -d Also, increase Postfix’s global verbosity: And once resolved, safeguard your configuration to ensure
The filter binary was recompiled but its dependencies (e.g., Perl modules, libssl) are now incompatible with the version Postfix is trying to run.
The key to resolution is identifying the specific transport (pipe, LMTP, smtp) that is failing, checking the system logs for underlying errors, and verifying that the external binary or socket is functional when invoked manually by the postfix user. Understanding the Error: What Does It Mean
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: