Cannot access site externally, but I can locally

I'm having a problem accessing an asp.net website application externally (i.e. outside of the firewall) but I can access it internally. I have checked DNS outside using DNS lookup tools and it resolves correctly to the IP address. There are 2 sites on the server that I have tested using host headers. One site works fine (the currently bound site) but the other does not and this is the site that I need to resolve. The website is www.hampdentech.com. Presently it is bound to one application which correctly displays the site. If I remove the binding from the current site and bind it to the site in question, the site cannot be located externally. However, if I go to www.hampdentech.com on the server itself, I can correctly navigate to the site and work with the application. This tells me that DNS is setup correctly and that it has something to do with the site itself. I'd like to debug this and have viewed the IIS logs as well as the event viewer but ...