Please go through the below settings to configure AT&T.
1. Description = Your Email Address.
2. Server Name = mail.bellsouth.net, or use the one in your email address (AT&T has many different domains)
3. Port = 25 or 100 (This was supplied by my ISP/Mail)
4. User Name and Password = Unchecked.
5.Connection Security = None.
How it will help you