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.
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: The key to resolution is identifying the specific
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. It tells you something is wrong, but it
The filter binary was recompiled but its dependencies (e.g., Perl modules, libssl) are now incompatible with the version Postfix is trying to run. dovecot unix - n n - - pipe
dovecot unix - n n - - pipe To (add v flag and increase verbosity):
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: