How long has it been happening?
Every couple of weeks I have this happen with Mail and .Mac and it goes away after an hour or two at the most (usually) Not sure why it does it for me but seems to correct it self after a bit.
I'm using comcast (jacksonville) and i have the following settings:
mail server: smtp.comcast.net
port: 25
Use ssl: yes (checked)
Authentication: none
user name/password: leave empty
Maybe it's different elsewhere, give the above a shot though.