Thunderbird deletes old messages without asking

I'm using Thunderbird 102.13.0. I recently noticed, that mail downloaded (via IMAP) to my Inbox, disappear, when they are more than 6 months old, and I really mean "disappear": They are not in the "Deleted" folder either. I checked the preferences whether there is a setting, which would purge automatic messages automatically, but I could not find any. What could be the cause of this behaviour?