I’d suggest opening a support ticket, we’ll have a look.
There actually is a proxy server in play, depending how you connect to webmail. If you connect via yourdomain.com/webmail, the client Apache has a proxy to the InterWorx Apache on port 2080 or 2443, depending how you connect.
One thing to check would be accessing webmail through the InterWorx ports directly, and see if there’s a larger problem. You can go to https://yourdomain.com:2443/webmail for example.