Hi - I am posting a solution to a frustrating problem that I had a few days back and how I solved it. This is one a bit technical (read: hard to find) so please bear with me. Problem: Authentication failure while connecting to imap.gmail.com Assumption: I am assuming that you have put the settings of the server exactly how Gmail requires you to give in Thunderbird. I am also assuming that the authentication dialog comes up and you are able to allow Thunderbird an access on the Google authentication page and yet Thunderbird responds with authentication failure. Also, I am assuming that there's no antivirus/firewall blocking communication with Gmail or Thunderbird app itself. Solution: Check of you have IIS or Apache or anything else running on port 80 - basically - if typing http://localhost on the browser shows something. You see, after authentication from Google, Thunderbird tries to go back to http://localhost - and it expects to find itself BUT it finds your already running web server itself. I resolved my problem by ensuring no webserver is running at the time of authentication and that solved my issue. Hope this helps someone. Nirav