Enter imap.mail.att.net as the incoming mail server for IMAP.
Enter smtp.mail.att.net as the outgoing mail server for IMAP.
Enter 993 as the incoming server port for IMAP.
Enter 465 as the outgoing server port for IMAP.
Choose the options of SSL as the type of encrypted connection.
Read more here:- https://www.datarecovo.com/att-email-settings-for-outlook/