HI there,
First we need to isolate the problem
Create a new user account called test and log in to it
Configure your bellsouth account in Mac mail and check if it works there
If it works in othe account then its definitely an issue with ur account then u need to back up ur mail account by using this:
http://support.apple.com/kb/HT2980#mail
then delete the com.apple.mail.plist under preferences and mail folder since u have already backed it up
Now open mail and re-configure ur accoutn again and see if it works
If the issue occurs with both USER account then u might have to re-install mail
Good Luck