I have that too sometimes. Not with .Mac, I have a different e-mail address. I always solve it by quitting mail.app, waiting a few minutes, then opening it again.
My guess is that it's because you checked your mail a little bit too often, and then the server decided that he'd keep you out for some time. I have set mail.app to check every 5 minutes for new mail. When I'm on the computer the whole day, I'll get the error on the end of the day? overload.
This is Mail.app's anemic way of saying it can't properly contact the server. It assumes that it's just a bad l/p combination and reports that as the error. Choosing to cancel and then reconnecting later will work.
Comments
My guess is that it's because you checked your mail a little bit too often, and then the server decided that he'd keep you out for some time. I have set mail.app to check every 5 minutes for new mail. When I'm on the computer the whole day, I'll get the error on the end of the day? overload.