Hi, AT&T has constant problems, see if you can use the same Login & PW in WebMail in a browser.
In Mail's Window Menu, choose Connection Doctor, any red dots for status, if so what is the exact message?
Then click the Show Details button & Check again.
The Password rejection can confuse people since it's a catch all meaning...
This Password, Username, Authentication method... is not recognized on this Port to this Server.
So since WebMail works it's not Name or Password, but one of the other ones.
The receiving email ports are:
IMAP is port 143
IMAP-SSL is port 993
POP is port 110
POP-SSL is port 995
Outgoing ports are...
SMTP and SMTP-SSL is on ports 25, 587 and 465. Port 587 has to be SSL, and port 465 is enforced TLS-wrapped and is generally used by Outlook users.