I started having problems with Mail last week (Os10.2.8). When I click on the program, it opens and the spinning circle let's me know that it is opening mail. However, I then get an incorrect username or password error, even though I have repeatedly confirmed that the username and password are correct. I have no problem logging on the net and viewing my email using the same username and password, Mail just cannot seem to log in to get it. This just started on Friday.
I'd write down all the account details, delete the account in Mail preferences, quit and restart Mail, and then re-add the account. Has to be worth a shot. :)