Or, maybe it's "none of the above" per "dshuford" in:
... asking her to try that workaround (mail.bellsouth.net, unencrypted, no auth), will post back with the results.
I assume they'll break that once they get outbound.att.net working for Mac Mail.app users, if there's any truth to it at all.
[Edited to clarify that the issue is said to be specific to Mail.app. Haven't tested personally from her machine, but I know everything's fine with other clients used from elsewhere.]