Assuming it's not something obvious like a mistyped password, a uppercase/lowercase problem, etc....
Sometimes this can be a sign of your ISP mail server being down for maintenance or misconfigured. Has the problem been going on for very long, and is there a way you can check on the status of your ISP's mail server?
I've noticed my OE 5.5 does this sometimes, too, and a few times it's actually been that the stored password became corrupted. Try retyping it if you haven't already.