How can I change the default listening port for Exim?
First and foremost, changing the port for Exim to something other than 25 will prevent email from sending and receiving without a smart host. This is how mail servers talk to each other.
Warning: If you block port 25, your server (and your users) will not receive any mail.
You can add an additional port through "WHM / Service Configuration / Service Manager" and find "Exim on another port."
To change the port that Exim starts on, navigate to "WHM / Service Configuration / Exim Configuration Manager." Then, open the "Advanced Editor" and look for "daemon_smtp_ports."