Postfix : Relay Access Denied

Root Cause

An error log appears in the postfix maillog during the email-sending process, this is because the network host/client-server of the email-sending application has not been added to the mynetworks configuration in postfix/main.cf

Solution

Open Postfix main configuration /etc/postfix/main.cf

$ nano /etc/postfix/main.cf

Add the host IP address on mynetworks conf

mynetworks = 172.0.0.1

Restart Postfix service

$ systemctl restart postfix