This could be gmail deliberately blocking you. Pop or IMAP mail account ? If POP : logon to webmail account via a browser and select to use 'less secure app'. If Imap are you using 'Authentication Method' = 'normal Password' or 'OAuth2' ? If 'Normal Password' then do same as for POP. If you subsequently swap 'Authentication Method' in 'Server Settings' and 'Outgoing Server (SMTP) settings to using 'OAuth2' then you will be diverted to a browser to login. This is gmail checking that you really are you. Then a token is downloaded into Thunderbird. From then onwards gmail will use that token instead of the password.