Mario, This is typically seen if you don't have the "Remember my password" (or something similar, I don't have Thunderbird on this box, but have used it tons) setting checked in your actual e-mail account settings within TB itself. If it's not checked, even if you entered it when setting up the account, it will be "forgotten" by design so that you have to enter it each time you need to connect to the server. Go into your account settings and double check on the page where your full login information is stored.
Brian
