Remove the passwords from Preferences (Options)/Privacy & Security/Passwords/Saved Passwords, restart TB, enter, when prompted, a secure mail key that is created on the AT&T site:

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

Thank you! Success!

Thank You Thank You

I reported a similar problem with my AT&T account using Thunderbird 78.4.3 on Windows 10, and switching to using an AT&T secure mail key (described above by sfhowes) solved my problem. For reference, you enter the secure mail key where Thunderbird prompts you for the account password; and then have the Thunderbird password manager store it for you, as you would for a password.

Been wrestling with it off and on all day, but all seems good now. Yay!

Thank You. I'm sure I read that and didn't understand I don't want Oauth I wanted a secure mail key. I did that and suddenly my mail is coming in. What's still strange about this is, only the two accounts that I logged in on the AT&T site, have changed to this. Old accounts that I never checked mail online, or logged in, using the AT&T site, have no issues. I'm making a text file for myself, with the link, because who knows what something will change.

I have over 20 assorted email addresses. Some for web services, one for family, a couple for business, another for photo work, another for... OK you get the idea. Only two of the 6-8 old Prodigy accounts flipped the switch and blocked me.

Thanks for a clear answer and the help from other people with the same issue. AT&T I spent close to an hour and they were no help.

BRAVO Mozilla Support

I was not able to find any kind of access to my password. I use the same password to access webmail, same email account name, same password. No thunderbird access. I tried deleting my Thunderbird account and setting up a new one with the same address and password. No dice--it recognizes that that account is in existence and can not let me have my email ...

sfhowes, thank you SO much! Solution easy to follow and worked for me. Frustration at an end!  :-)

Awesome sfhowes! I was about to run from the room screaming when I found this. It fixed it! Thank you so much!