Thunderbird can be used as a client for an AT&T email account. Thunderbird will download messages from the AT&T server and store them on your local system. Similarly, Thunderbird can be used to compose messages which are sent through AT&T's mail servers.
The following are the settings required for AT&T connection.
Description = Your Email Address
Server Name = mail.bellsouth.net, or use the one in your email address (AT&T has many different domains)
Port = 25 or 100 (This was supplied by my ISP/Mail)