Is your account set for Oauth2 authentication? If not, it's now required. The password you use for web access no longer works on an email client, such as Thunderbird. You need the new authentication. Here are the basics: - change your security authorization to Oauth2 (do this for the server settings and also for the Outgoing (SMTP) server - two places - remove the password entries for the account (remove the entire entries both incoming and outgoing) - check to ensure that TB is set to accept cookies - Google will then step you through the authentication process for your PC. (If your TB does not offer the authentication option of Oauth, then you need to upgrade your version. )