Go to options of "Settings" after unlocking the system.
Choose the options of "Mail, Contacts, Calendars" from the drop-down menu.
Choose the options of "Add Account" with the options of drop-down menu.
Select "Other" from the drop-down menu.
Select the options of “Add Mail Account” and fill in the required information, including your full name, email address, password.
Choose the options of preferred choice if you want to sync both Notes and Mail.
Enter the button of save.
Follow above given steps to Set up ATT email in iphone.
See Also this post:- https://www.datarecovo.com/att-email-settings-for-outlook/