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:
dovecot unix - n n - - pipe To (add v flag and increase verbosity): sudo tail -100 /var/log/mail
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. smtp) that is failing