Usually indicates an issue at the mail server end of things, however a common thing to happen is for the username/password requester to pop up and the user pressing something on the keyboard thus changing one or the other, resulting in it never working again. Double check you are using the correct details, if it still fails go find the service status page for your mail provider and see if any faults have been reported. If none and it's still doing it you'll need to give them a call to let them know.