Mail Failure

Mac mail will not log in to email server (ATT). I have a desktop iMac, upgraded to Big Sur two or three months ago, and am now using I think most recent version of Mail (14.0). Working fine since then, until now. Four days ago (a Monday) Mail would not connect to server (message is "login failed"). Mail remains "offline." All settings are still correct (imap, smtp). Two calls to customer service at ATT achieved nothing. No login since. For the first three days, I could log into emails at the ATT website via Safari, although it looked like it was passing me through to Yahoo. Today, I cannot log on through ATT, but I was able to log in directly at Yahoo. Handling mail through the web is tedious and not my desired option. Does anyone know what the heck is going on? For more than twenty years, no email problem--now all of a sudden it doesn't work! I surely wish to avoid switching to a different service, such as gmail, since my email address is the user id for countless accounts, not to mention on all my business cards, stationary, etc. I see lots of frustrated mail users on online forums, but no one addressing this particular problem. Seems like something is going on at ATT/Yahoo that they refuse to acknowledge. Anyone know? Thanks.