OWA problems

I am using Exchange server 2003. I am able to access OWA from the indside of my domain using http://server_name/Exchange. However I cannot access from the outside. I have used Microsoft KB article Id: 301428 “Troubleshooting Outlook Web Access from an IIS Perspective”. Everything in IIS is as it should be yet I don’t even get the “loading” page. What I do get is the following, Background:
“The gateway could not retrieve the requested page”. I could use some help!. In general my Exchange runs fine except for the access problem. Thank you