i just migrated over from microsoft entourage 2008 to apple mail. i never had issues sending from entourage, but in apple mail I couldn’t send. so i fiddled with the settings and i was able to determine the following.
In Entourage – i just used my username (for example “jdoe”) for connecting to the mail.bellsouth.net and it worked perfectly in entourage
not so in apple mail
In Apple Mail – i thought instead of just having “jdoe” as the user, i should try using “firstname.lastname@example.org” and it worked!
so for some unknown reason, when connecting to bellsouth (mail.bellsouth.net), apple mail needs your complete email address.