A few years back, a very skilled programmer created a proxy script that makes is possible for users behind firewalls to access the ports servicing cPanel, Webhost Manager, and Webmail. You can install or download the proxy here:

http://cpanelproxy.net/

http://cpanelproxy.net/autoinstall

There’s a small issue with the webmail proxy on cPanel 11 servers, but there’s a fix located on the SourceForge site to allow it to work.