Allowing Yahoo web Mail Only

I am using ISA SERVER 2004, i just want to allow my users to check there yahoo web mail accounts, no Yahoo Messenger.

i this regard i made a rule that allows access to URL *.yahoo.com, *.mail.yahoo.com with protocol HTTP, HTTPS.

Yahoo web mail page is not displaying properly, message is not displayed and the page setting is out. though it opens the page

If any one can help