Do you have a 3rd-party password manager that is part of some security programs? Unless you were entering the email password each time TB was opened, it must have been stored somewhere.

I must enter my password each time I startup Tbird. I am able to login to and get mail directly from the ISP.

So, you restarted TB, and when prompted, entered the new password, and received no error message?

Here is the msg I get each time I try to login. Sending of password for user s.kiernicki@att.net did not succeed. Mail server inbound.att.net responded: Server error - Please try again later.

Here is a possibility: AT&T now requires secure mail keys in place of passwords, when a password is changed, although regular passwords still work if they were created before the policy change.

https://www.att.com/support/article/email-support/KM1240308

If you create a secure key, you'll probably want to have it stored in Saved Passwords, as the key is a long string of random characters. Check the box in the window when you're prompted for the password.

Of all the people I have talked to about this issue you are the first one to mention the secure email key. I set up a secure key months back in response to an email from att. Great work friend! And thank you very much for jogging my memory about this. I think I will now try to head in that direction for a fix. Do you know if any new releases of Tbird will support OAuthorization?

TB currently supports OAuth2 for gmail, Yahoo, AOL, Yandex, mail.ru and possibly a few others. Some services based on Yahoo, such as Frontier, Verizon and Sky, can use OAuth2 if they can access accounts on the Yahoo servers, e.g. imap.mail.yahoo.com and smtp.mail.yahoo.com. I think AT&T is Yahoo-based, but I don't know if you can use the Yahoo servers with an att.net address. If you can access your att.net account by logging into the Yahoo webmail page, you probably can set up the account in TB with OAuth2 on Yahoo servers.