Had a problem with email disappearing from a Thunderbird 1.5/imap/Dovecot installation. My inbox was not retaining email. Sometimes it would appear for a few seconds and then disappear. At first I was running around blaiming the software. However, it turned out to be a rogue copy machine/scanner that had been set up to use my email account as a return address for sending out scans via email.
Moral of the story? Check your logs when stuff like this happens. Robust software like Thunderbird and Dovecot rarely just stop working for no reason.