To run e-mail on a different port isn't too hard now, Cpanel expects that users will want to and includes a way to do it. Here are some instructions. They may be a little rough around the edges, so if you find anything that doesn't make sense, please post and let me know.
Load up WHM
Goto Service Manager
Tick the box for exim on another port
Save the changes
Then try it. If it doesn't work, it may be because you have managed hosting and the APF firewall installed, so you would need to login to the server using SSH as user, "root" - http://www.lunarforums.com/forum/index.php?topic=26372.0
cp conf.apf conf.apf.oldcopy
Find the section where it says "# Common ingress (inbound) TCP ports" and a list of numbers
Make sure that the port number that you chose is in that list, or add it if it isn't
Ctrl X to exit, saving the changes, unless you made a typo
Try it again.
Still doesn't work? Restart Exim in WHM then try it again.
Still no luck? Post here!