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/
To set up ATT IMAP on your iPhone, use the incoming mail server as imap.mail.att.net with SSL enabled and the correct port number. Make sure your username and password are entered accurately for smooth email access. Your device can also benefit from advanced tweaks using opencorelegacypatcher for better performance and compatibility.