Bellsouth e-mail address has stopped working

My e-mail has suddenly stopped working, even though all settings are correct.  ATT my internet provider tells me it is a compatibility problem with Outlook for Mac 2011. I have an old Bellsouth.net address. I have tried various pop and smtp configurations suggested by ATT, but nothing works.  I get a cannot send mail error, please enter username and password, but they are correct and even on re-entering the same error comes up. Do you have any ideas? Thank you Florida Blade.  You can reply on another e-mail address that does work. *** Email address is removed for privacy ***