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

https://www.att.com/support/article/u-verse-high-speed-internet/KM1010523/

The email settings are the same for the address that works and the one that doesn't. What do you mean by "Create a secure mail key on the AT&T site?" More specific instructions for that part would be appreciated. Thanks.

There is a link on the page I cited that explains secure mail keys. An old account may still work with a regular password, but if you change that password now, after the secure key policy was introduced, it won't work.

I will check it out ASAP Thanks for the info.

Unfortunately, AT&T no longer allows you to follow their directions. You can no longer see your email accounts from the profile page or select a secure mail key from any of their options. I logged into my account but all links led no where. What a surprise. AT&T makes everything more complicated. I tried calling support and only got a recording that did some trouble shooting which didn't find any problem and didn't allow me to respond. Fortunately, I'm using Gmail for most accounts.

They must have changed their key procedure, since this has been an issue for several months, and most users have been able to create a key (just search this forum for secure mail key). If you were able to connect to your bellsouth account through Yahoo servers, pop.mail.yahoo.com, imap.mail.yahoo.com and smtp.mail.yahoo.com, then you could apply OAuth2 authentication, instead of 'normal password', and a secure key would be unnecessary.