You don't give us much to go on here, but such error messages usually indicate that there's a problem on the email service's end, or you have Thunderbird set up with incorrect server settings. If it's their problem, it might be worth a call to them to see if there is a service outage. You should also check with their tech support or help web page to confirm that your server settings are correct. Many ISPs are outsourcing their email service to other companies like Yahoo, and I am noticing that they sometimes don't bother to tell their existing customers! If you tell us who is your email provider and your server settings (screen shots would be OK,) we might be able to advise you further.