I'm sure it has issues - heck, I've even filed bugs against it personally - but your comment doesn't at all resonate with my experience as a user.
The open bug I linked above is 13 years old, so maybe it isn't.
I know of some that I trust not to lose my mail. I know of some whose bugs don't worry me as much as stuff like this:
https://bugzilla.mozilla.org/show_bug.cgi?id=462156
> your comment doesn't at all resonate with my experience as a user.
Okay? I just assume that you don't believe nobody loses data to this program simply because you have not lost data to this program.
Much easier to backup if your email is in one place and running even a local IMAP server is better than storing it in an email client.
The 13-year old bug I linked to involves IMAP mail handling, I believe. I haven't dug too deeply into these bugs because, honestly... I'd like to be able to use Thunderbird for a couple of things, but mail handling doesn't need to be very screwed up for me to avoid the program. A 13 year old bug involving data loss is enough to make me avoid the program, even if it would not appear to effect my use case.
> Haven't lost anything
I guess I can understand how others might view it as "just email." I won't argue with the implied "I've never lost data to this program so it must never happen" argument, but I wonder how you know you've never lost anything. I feel like I'm capable of not noticing a data loss bug involving email, or not noticing until I need something and it is not there.