How can I (or my customers) access webmail, cPanel, and WHM on port 80? I am behind a strict firewall

We have built functionality into cPanel to use cpanel.domain.com and webmail.domain.com over port 80, so that you can still access cPanel and your webmail behind a proxy server. Simply replace domain.com with the name of your website.

This functionality can be turned on and off in the Tweak Settings menu of WHM. The specific setting is called Automatically create cpanel, webmail, webdisk and whm proxy subdomain DNS entries for new accounts. When this is initially enabled, it will add appropriate proxy subdomain DNS entries to all existing accounts. Use /scripts/proxydomains to reconfigure the DNS entries manually.