When you changed the password, did you generate a secure mail key, remove the old password in Saved Passwords in TB, restart TB, and enter the key when prompted for a password?

https://www.att.com/support/article/dsl-high-speed/KM1010523/

Thanks - that worked.

I remember the secure key from several years ago, but also seem to recall that I couldn't get it to generate a secure key for all the profiles. Yet all the profiles worked.

Do I have to keep the secure key as my password? Or since TB is now working can I use my own password?

Thanks again.

Use the key in TB or any other mail app, but use the password when accessing the account in webmail.

Thanks -

Another question, what's the purpose of choosing a nickname? Everywhere I searched said basically the same thing, "Enter a nickname for the secure mail key to make it easier to recognize."

Since I have to cut/paste the secure key into Thunderbird, when is the nickname used?

I think the nickname is only used on the AT&T site, and is useful if you have several accounts, each with its own key.