Can I block email ports for one domain?

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):

• https://docs.cpanel.net/whm/security-center/host-access-control/
• https://docs.cpanel.net/knowledge-base/general-systems-administration/how-to-configure-your-firewall-for-cpanel-services/84/