Append one or more “-v” options to selected daemon definitions in /etc/postfix/master.cf and type “postfix reload“. This will cause a lot of activity to be logged to the syslog daemon. For example, to make the Postfix SMTP server process more verbose:

/etc/postfix/master.cf:    smtp      inet  n       -       n       -       -       smtpd -v

To diagnose problems with address rewriting specify a “-v” option for the cleanup(8) and/or trivial-rewrite(8) daemon, and to diagnose problems with mail delivery specify a “-v” option for the qmgr(8) or oqmgr(8) queue manager, or for the lmtp(8), local(8), pipe(8), smtp(8), or virtual(8) delivery agent.