Although the CommuniGate Pro mail server is not officially supported by cPanel, you may be migrating to cPanel and want to move over hundreds of CGP mail accounts. Of course you won't be able to offer cPanel's mail services to your users (since you're providing vastly superior mail services through a 3rd-party product), so you'll need to disable them in WHM. You'll also want to turn off POP and IMAP services in cPanel, since CGP will be providing them on its own.

Install CGP normally and move your /var/CommuniGate mail database into place (one of the beauties of CGP is its easy portability between hosts, even on different OSes).

Now here's the tricky bit. You'll notice in CGP's logs that it isn't able to open several of the ports needed for standard mail services (POP, IMAP, secure IMAP, etc.) This seems strange since you've already disabled those services. But run:

netstat -plven | grep 143

for example, and you'll see that stunnel is hanging onto this port, 993, and others. Odd. Run:

ps aux | grep stunnel

and you'll see :

/usr/bin/stunnel-4.04local /usr/local/cpanel/etc/stunnel/default/stunnel.conf

so now you know where stunnel's configuration file is. Strangely, stunnel is not started from /etc/init.d or etc/rc.d/rc3.d/* Open the file and you'll see that its configured to use these ports. Comment out those lines and restart stunnel, restart CGP, and CGP is happy. But... now WHM doesn't work! I have no idea why WHM wants these ports open, but it turns out that it does need these services running on *some* ports. So choose some random unused ports to give to these services instead. Here's what my stunnel.conf looks like now:

[pop3s]
# Commenting these out so CGP can use them.
# cpanel WHM needs these to be running for some reason,
# so choosing random unused ports
#accept = 995
#connect = 110
accept = 998
connect = 112

[imaps]
# Commenting these out so CGP can use them.
# See above.
#accept = 993
#connect = 143
accept = 997
connect = 146

Of course these ports will still be blocked by your firewall (assuming you've configured your firewall correctly), so it's not unsafe simply to run unused services on them. Reboot the server and you'll find that CGP gets all the ports it needs on startup, and cPanel / WHM still work properly.