Why can I continue to receive but suddenly cannot send out messages on Windows Live? They just sit in the Outbox.

My outgoing email messages just recently started sitting in the Outbox and don't clear.  I get a pop-up screen that asks for my user name and password, and when I input them, the pop-up screen just keeps popping up (I know that my user name and password are correct.) The local cable company that is my internet provider has been unable to find a solution to the problem.  They suggested I call Windows Live Support, which has not been helpful at all, except to suggest I try the community route.