Server Webmail not working

Question

Have run into a issue with my mail server. I can access webmail from internally but not from any compter outside of my local network. When accessign it from outside it opens to the webmail logon screen and when a user enters their user id and password it just sits and eventually comes back to the logon screen again. This stopped being accessable sometime around the time we updated the server to 10.7.3. No Changes to the firewall were made only the normal updates to the server OS. This has been working for almost a year (originally running on an Old Mac Mini with Snow Leopard Server)
In troubleshooting I have tried making the Server a DMZ host in the firewall, with no success. Also in an attempt to try andsee if it is an issue with port blocking by my ISP I connected an external device via a VPN (Hosted on same server) and still had the same results.
Current Setup:
Small Home Office
Mac OS X Lion Server v10.7.4
Mac Mini is being used as the server (Current Generation)
ISP is Charter Communications

Answer 1

Have you tried using the network utility app yet?  Use the lookup tab and check to make sure your domain name matches your ip address and see if you can look up your mx record.  So what I usually do is
1.  Check server.mydomain.com  - see if it is linking to your ip
2.  check ip and see if you get the reverse look up
3.  check your mail server (mine is mail.mydomain.com) it should link back to your fully qualified domain name. 
I just did a fresh reboot and I was having the same issue.  So in the server admin (download from apple - http://support.apple.com/kb/DL1419) under:
DNS ->  Zones create a CName (alias) under your fully qualified name for your mail server (MX record). 
After that my email was working fine.
Hope this helps