On your computer, go to the options of settings.
Look for the option "mail, contacts, calendars" and select it.
Select the options of Add Account from the drop-down menu.
Now you'll see a option called "others," which you should select it.
Enter your ATT email address, full name, password, and a personal description by entering the button of Add Mail Account.
If you like, you can now sync the notes and mail options. Simply enter the options of sync.
Enter the button of save.
Read more here:- http://datarecovo.populr.me/how-to-set-up-att-mail-on-iphone
See Also this post:- https://datarecovo.educatorpages.com/pages/complete-guide-to-att-email-setting-on-iphone