postlog
This command allows an external program to write to the mail log. This is what you would use if you had a shell script that you wanted to write messages to the mail log.

# postlog testing postlog

postfix/postlog: testing postlog

Now check the log.

# cat /var/log/mail.log | grep testing

Jul 3 23:49:02 testub postfix/postlog: testing postlog

Send a “testing” message and then review the log.