Use the server settings given here https://www.att.com/support/article/email-support/KM1010523/ and a Secure Mail Key instead of your regular password.

Does the Secure Mail Key replace the password? In other words, the Secure Mail Key IS my new password?

Stans, I am using the server settings from the webpage you referenced. I also am using the Secure Mail Key. I am still getting the same error message from Thunderbird. Any further suggestions will be appreciated.

Did you remove the old passwords from Options/Privacy & Security/Passwords/Saved Passwords, restart TB, and enter the secure key when prompted?

Stans, I am using the server settings from the webpage you referenced. I also am using the Secure Mail Key. I am still getting the same error message from Thunderbird. Any further suggestions will be appreciated.

Make sure authentication method is set to "Normal Password" in Account Settings for both incoming and SMTP server.

For some reason, my server setttings for authentication method was set for "OATH2". When I changed it to "Normal Password" my Thunderbird started working normally i.e. no Authentication error message. Thanks for your help and suggestions!

It did this to all of my gmail accounts after update, which need to be OAUTH2. Changing it away from OAUTH2 won't work... any idea what's going on with how to fix gmail accounts that get this error?

Edit: I decided to remove and re-add the account, when I did so, I had to reconnect thunderbird to gmail as an "allowed app" again, so the previous "allowed" connection to Thunderbird was somehow insufficient or wiped out. So now I have to remove, re-auth, and re-add all my accounts, which is a pain in the butt with 2 factor auth set on all of them. If this happens again, thunderbird is going in the bin.

It did this to all of my gmail accounts after update, which need to be OAUTH2. Changing it away from OAUTH2 won't work... any idea what's going on with how to fix gmail accounts that get this error? Edit: I decided to remove and re-add the account, when I did so, I had to reconnect thunderbird to gmail as an "allowed app" again, so the previous "allowed" connection to Thunderbird was somehow insufficient or wiped out. So now I have to remove, re-auth, and re-add all my accounts, which is a pain in the butt with 2 factor auth set on all of them. If this happens again, thunderbird is going in the bin.

Removing and re-adding the accounts is not necessary. You could have fixed this easily by simply deleting saved passwords for the Gmail accounts then restart Tbird to trigger the OAuth2 process for them.

I have tried all these methods (I think!) and none of them worked.

In particular, trying to set "normal password" fails totally (I am not allowed to access gmail).

The OAuth2 method work most of the time (about 70% of my attempts to read mail or send mail), but this is hardly satisfactory!

Here is a typical episode (about once or twice a day on each of my computers):

1. Opened Thunderbird.
2. Waited for tbird connect to gmail imap server: "sending login information..."
3. After about 1-2 minutes I get an error message "authentication failure..."

I work on a KDE desktop KUbuntu 20.04. Thunderbird v 68.10.0.

stuck again!