- Login to the server over SSH
- 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