It looks like you're trying to set up a POP account with an IMAP server. When you add a gmail account to TB from File/New/Existing Mail Account, the setup routine automatically enters the settings for IMAP. If you want POP, click Manual config. and change IMAP to POP, and use pop.gmail.com for the incoming, smtp.gmail.com for the outgoing. With TB 68.5, OAuth2 authentication is available for both the incoming POP and outgoing servers. http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite