The current version of Thunderbird 1.5.0.8 has a major bug that causes some mail that is downloaded from the server to disappear. Thundebird displays the number of new mails that is in a folder but if you click in that folder the new mail that should be there is not. The first time I have read about this message was on the mozillazine forum where a user named soulek described the problem that he encountered after upgrading to Thunderbird 1.5.0.8.

