can see IIS4.0 web site in Intranet but

I have a NT4 server with proxy, IIS and exchange. I built a web site in IIS and I can see the web in my Intranet. However, I cant browse it via internet. Nevertheless, I can surf the internet via the proxy and the router in my Intranet.
So where should I start to troubleshoot? or anyone has any idea of such problem?