Go to the options menu and Choose the options of settings.
Choose the options of add account from the passwords & account menu.
If the ATT option is not available, choose the options of others.
Add mail by clicking on it and filling in the necessary details.
Fill out the IMAP settings that have been presented above by clicking on IMAP.
After that, enter the button of next.
Enter the button of save.
The outgoing server port should be 993.
Find the outgoing server alternative and use port 465 as the server port.
Enter the button of done and restart your iPhone.
Read more here:-
https://www.datarecovo.com/how-to-set-up-att-email-on-iphone/
See Also this post:- https://www.datarecovo.com/fix-att-email-not-working-problem/
Set up Outlook 2016 for AT&T email
Use these steps to set up your email for the first time or on a new computer.
In Outlook, select File > Add Account.
Choose Manual setup or additional server types.
Select Pop or IMAP.
Complete your User Information:
Enter your name as you want it to display to others.
Enter your full AT&T email address.
Complete the Server Information:
Account Type - Select IMAP or POP3.
Incoming mail server - Enter imap.mail.att.net (IMAP) or inbound.att.net (POP3).
Outgoing mail server - Enter smtp.mail.att.net (IMAP) or outbound.att.net (POP3).
Complete the Logon Information:
User Name - Enter your full email address.
Password - Enter or paste your secure mail key.
Check Remember password and Require logon using Secure Password Authentication.
Select More Settings.
Complete Outgoing Server info:
Check My outgoing server (SMTP) requires authentication.
Select Use same settings as my incoming mail server.
Complete Advanced info:
Incoming server - Enter 993 (IMAP) or 995 (POP3).
Use the following type of encrypted connection - Choose SSL.
Outgoing server - Enter 465 (IMAP and POP3).
Use the following type of encrypted connection - Choose SSL.
Select OK and then Next. Outlook will test your account info. If everything passes, select Finish.
Regards,
Bruce