It MIGHT be YOU and not ATT!
I had the same problem with my sbcglobal.net accounts not sending email on my new iPhone 5. The instructions for properly setting up an iphone for SBC Yahoo are hard to find. Apples instructions are incomplete and their preset Yahoo settings don't work. In fact they assume you want to set up an IMAP account instead of POP3. So I entered everything manually. (I forget exactly how I got the option to select POP3 as the type of account but I recall having to type in a fake email account address and then going back and correcting it AFTER the option to choose POP3 came up and was selected).
Everything seemed OK for recieving email but I still could not send them from my iphone. Incoming mail port for sbcglobal is 995. (In Account-->Advanced-->Server Port).
Solution, Go to: Settings -->Mail, Contacts, Calendars-->Accounts (select account)-->Outgoing Mail Server-->Select/Tap where SMTP server is listed
Your Primary server will be listed at the top (same as you just tapped on). I have 3 accounts set up on my iPhone for SBCglobal.
Under Other SMTP servers I had to turn ON (Toggle Server On at right) for the Outgoing Mail Server that corresponded to this user account. Password was already there, Use SSL was ON, Authentication is set to password, server port 465 (for my outgoing sbcglobal).
Interestingly I did NOT have to do this with my iPhone 4S and mail still sent properly. But everything in Mail works now on my iPhone 5.