There are some suggestions here: https://support.mozilla.org/en-US/kb/cannot-send-messages Are you getting error messages when you send mail? There's an article here which summarises the stages involved in connecting to your outgoing server: http://kb.mozillazine.org/Connection_errors_-_SMTP You'll see that there's a series of 'status messages' that you should see in Thunderbird as the connection proceeds. Try sending a message and watching closely -- which status messages do you see and at what point does the process terminate? That may give a clue to what is going on.