-Access to less secure apps is already turned on for both accounts.
 -I changed authentication method to OAuth2 for both accounts.
 -I have updated to the most recent Thunderbird version: 78.8.0

I just added 2 gmail accounts yesterday to Mozilla Thunderbird. I'm fairly new to Thunderbird, but I was pleasantly surprised that all went well, and I received/sent mail from/to those accounts. Today I sign into Thunderbird, and it retrieved the emails fine. However, it would not send emails. It kept popping up a box to enter credentials for that gmail account. I entered the email address, but it will not move past that screen. It just keeps reloading the "Enter Email or Phone Number" screen for gmail. What I have already done (after researching): -Access to less secure apps is already turned on for both accounts. -I changed authentication method to OAuth2 for both accounts. -I have updated to the most recent Thunderbird version: 78.8.0 I have restarted Thunderbird after doing all of this, and each time I try to send the draft, the screen to enter credentials pops up again, and the loop continues, never letting me enter the password. Now it will not even check my gmail email account for new mail without that screen popping up. Please help! I don't know what changed.