Managing ports through the cPanel software out the box would require the functionality of a Firewall, and the cPanel software bundle does not include any iptables management services.
At this time, there does not appear to be a working method to restrict port(s) on a per-domain basis. With that in mind, this functionality is best achieved by restricting mails port(s) access on a per-IP Address basis instead. This can be accomplished through the use of the Host Access Control feature within WHM. To access this page, you'll need to navigate to:
[1] WHM interface
[2] Security Center
[3] Host Access Control
This feature allows for IP based restrictions to compatible services on the system.
Relevant Documenation(s):