I'm getting messages from MAILER-DAEMON user.
How can I disable it?
The Mailer-Daemon alias is used by sendmail in order to notify the postmaster (or anyone else you defined) on problems in email.
If you have no interest in being informed on such errors, redirect the Mailer-Daemon alias to /dev/null or to any other convenient path.
Example: MAILER-DAEMON: /dev/null