Microsoft Exchange Web Server Problems

My company is running an Exchange Server on Windows 2000 and I am having dificulties reaching the address for the web server from any connection outside of our ISP. Internally (on the local network) and from other sites on the same ISP I run into zero problems, but when trying to access the web server from another connection (ie. a Dial-up) it will not authenticate the user. But when you are in terminal server mode remotley it seems to eliminate the problem. We have a static IP on a cable internet connection. Any suggestions would be appreciated.