You may have requirements to add an additional SMTP port for authentication. This port will allow both STARTTLS and plain text authentication.
1) Access WHM and go to "Service Manager".
2) Scroll to "Exim Mail Server (on another port)" and fill the radio buttin to enable the setting and to monitor.
3) Then add a port like 588.
4) Next scroll down and click save.
NOTE: If there is a firewall including local and/or external firewalls then this TCP port will need to be opened inbound to the server.