How to enable 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, run:

# postconf debug_peer_list=""

Edit the /etc/postfix/master.cf config file, add -v right after "smtpd" in the file, like the following:

smtp inet n - n - - smtpd -v

Apply the new settings:

# service postfix restart

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