This happened twice, I installed thunderbird last night as I bought a new hard drive I did a fresh install. I set up one pop account and instructed it to leave messages on the server and to delete them only if I delete them. Then I noticed I was missing messages that are (approximately) more than a month old. Then I checked the web mail for that server just to be sure and really email was deleted.
Couple of months ago the same thing happened. I checked this with another account and the same thing. All email older than approximately one month is deleted.
WHY IS THAT?