Authentication failed – invalid credentials

Every so often i get a pop up window in Outlook that says "Mail could not be received at this time" with a message saying that authentication failed due to invalid credentials.  However, it keeps sending and receiving email just fine, these annoying messages keep popping up every second or two and prevent me from accessing email.  I've tried resetting my password, but that doesn't stop them from coming.  Has anyone else had this problem, and how do I stop it from happening?