I like Thunderbird, but several months ago a strange bug happened that caused one sent message to overwrite another sent message (IMAP account). I've noticed only because I needed to resend contents of both messages, and I was really surprised to find that Thunderbird overwrote contents of one message with another (Subject of the overwritten message was intact).
It was a one time bug and it might be caused by my IMAP provider, not Thunderbird, but still I'm worried. I keep almost all my emails, dating back to middle of 90s (although in different formats and accounts), and I would like to be 100% sure that all of them are intact. Now I don't have that peace of mind.