Are you attempting to connect to an ISPs' dedicated mail servers? Or are you trying to access hotmail or what have you?
If the former, then it may be that your ISP has had a little problem. Try sending them a message via their webpage or webmail. (You could be really radical and telephone them) This one is not that uncommon. If they change something at the server end, some of their users may find that a previously stable email account hiccups until it is reset from the far end.
If you are accessing a free or global email account, then further details might help us help you.
Conan