Hotmail could not work

I am using an internet proxy server so that other users get internet connection using only one internet account. When all the users on the network use internet, there is not problem any other site except WWW.hotmail.com site. When a user opens this site he/she gets the first screen and when he/she enters user name and password and press enter, it automatically brings a screen stating the server could not open the page. Don't forget that this don't happen on the proxy server itself. I couldn't figure out the problem.