Select an IMAP server.
imap.mail.att.net should be used as the incoming server.
993 is the incoming port.
SSL should be used at all times.
smtp.mail.att.net should be used as the outgoing server.
465 is the outgoing port.
See this post also:- https://datarecovo.puzl.com/_news/How-to-Add-and-Setup-AT%2526T-Email-on-iPhone%253F/299136
https://datarecovo.educatorpages.com/pages/att-net-mail-settings-for-outlook