Create a secure mail key on the AT&T site, remove the passwords from Saved Passwords in Options/Privacy & Security, restart TB, enter the key instead of the password when prompted.

https://support.mozilla.org/en-US/questions/1313640

I added a secure mail key AT&T removed the old password, the issue is there was nothing in Thunderbird to add the new password it removed my account from the password list and does not give me an option to add a new password i.e the secutre mail key.

After the passwords are removed and TB is restarted, you will be prompted to enter a password at the next connection. Enter the key instead of the account password.

There was no prompt to enter a password when I opened up a new connection to TB all I got was the same error message in my original question.

What is the antivirus? If it's not Windows Defender, remove it temporarily. If you're using a VPN, disable it during mail operations. If you have an external app managing passwords, possibly part of a security suite, disable its connection with TB.

I only have Malwarebytes forever and it does not effect my other two email accounts. I am no using a VPN and no external app managing passwords. My TB is on a desktop computer.

What do you have for the server settings? They should be inbound.att.net on 995, SSL/TLS, normal password; outbound.att.net on 465, SSL/TLS, normal password.

inbound.att.net on 995, SSL/TLS, normal password.

After checking the mail many times I finally got the prompt to fill out my new password. I entered my new mail key and got the same issue. Same error message: Sending of password for user [email]@att.net did not succeed. Mail server inbound.att.net responded: Server error - Please try again later.

After checking the mail many times I finally got the prompt to add a password and entered the mail key. Still not working I got the same error message: Sending of password for user [email]@att.net did not succeed. Mail server inbound.att.net responded: Server error - Please try again later. Again issue with TB.