This could indicate that you did not use POP for retrieving messages before trying to send, which is some way of spam prevention at your ISP. Does it happen after getting email as well? Or are you trying to use a SMTP server without "popping" at their server, e.g. because you disabled automatic retrieval? Another option would be that your SMTP settings changed in Thunderbird, or your ISP requires authentication where it did not previously (though that would be rare these days.) Make sure "Authentication method" is set to "Normal password" in the SMTP settings, or as provided by the ISP.