How to enable or disable Postfix debug mode?

Login to the server over SSH

Execute the following (specifying the required IP addresses and/or recipient domain names to monitor instead 203.0.113.2 and example.com):

# postconf debug_peer_level=4

# postconf debug_peer_list=203.0.113.2,example.com

Note: No spaces between values in debug_peer_list are allowed.

To disable debug via postconf, run the following command:

# postconf debug_peer_list=""

To enable Postfix debug mode via the master.cf file, you should open the /etc/postfix/master.cf config file by using your favorite command-line text editor

Add -v (for verbose) right after smtpd, as shown in the following example:

==========================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (no) (never) (100)
# ==========================================================================
smtp inet n - n - - smtpd -v

Note: To disable this Postfix debug mode, simply remove the -v you have added, save the changes and restart Postfix

Apply the changes by restarting Postfix via this command:

# systemctl restart postfix

Note: Postfix restart will lead to email service downtime for a while. Consider doing it during less active working hours.