1. Login to the server over SSH
  2. Execute the following (specifying the required IP addresses and 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

To disable:

#postconf debug_peer_list=””

Source:
https://support.plesk.com/hc/en-us/articles/115000254594-How-to-enable-Postfix-debug-mode