Hello
If you are using a proxy server then 1st you should need to configure all protocols with your proxy server.
2ndly if you want to share your internet connection then you must stop the WebSite serevices from your Administrative Tools.
just go to IIS Services on your server and stop all the services except smtp then your internet will automaticaly share with your clients.
if its will not work then contact me i will tell you all the details hmyasir@hotmail.com, punjabina4you@yahoo.com