If I remember correctly (I had AT&T for quite some time), you'll need the address of their SMTP server, you'll want to use port 465, SSL/TLS authentication and normal password. Your user name is your e-mail address, your password is your e-mail password; something like firstname.lastname@example.org
or something similar to that, mine used to be email@example.com
(one of the Baby Bells). Those values may vary, though but they seem to be pretty common for most SMTP servers nowadays.
Best place to check is their web page help section, there's a section there on setting up mail (or at least there used to be!).
Hope this helps helps.