See Automatic Conversion of Google mail accounts to OAuth 2.0 Authentication

Thanks Stans

I followed the instructions and things seem to work. However, I would like to know how my main email account can be applied only to Thunderbird and deleted from Gmail, if possible. Thanks for your help.

See https://www.wikihow.com/Delete-an-Account-from-the-Gmail-App

Other tutorials are available here https://www.google.com/search?q=remove+account+gmail+app

Stans

Thunderbird is still not receiving most messages. I have checked my account settings. How can I enable Thunderbird to receive on my account only, please?

Thunderbird won't receive messages for an account that you haven't setup in Thunderbird. You have to deliberately setup that account in Thunderbird, before it can connect to it and retrieve messages. If you have Gmail account A and B, and you've setup account A only in Thunderbird, there is no way it will receive messages for account B. Are you saying that is exactly what is happening here?

Hello tmillard

what you need to do is to configure your Samsung mail app. Someone (could this be you ?) has configured this application to have a Pop3 setting for the Gmail account you seem to use. At a minimum, you should go to the settings for this account and set autodelete from server to 'never'. Better would be to remove the account and recreate it as an Imap account, it would allow you to quickly see new mails when you are not at your PC. In all cases, the best place to find help about configuring Samsung mail app is not here. Not everyone has a Samsung phone. I don't have one.