Contact AT&T and ask them for information regarding their SMTP servers. It's the same as setting up Outlook on a PC... you need to enter a valid SMTP server to send the mail. There is one caveat... you must use IP address... you cannot use the domain name for the Server.
Checkout this
article regarding setting up Outlook on AT&T DSL. I didn't read the entire article, but it looks like what you need.
