I have in Thunderbird my email from Jan 2015 - now. I suddenly noticed that in several folders emails have been deleted. E.g. in one folder all emails before Nov 11 2017 have disappeared (not just what is displayed in Thunderbird, they have disappeared from the data files as well. In another folder all messages from before July 13 2017has disappeared. In other folders no messages have been deleted (e.g. the messaged from Jan 1, 2015 - now are all still there.). I know for sure there were many emails (hundreds) in the folders that now only go back to a date last year (dating from Jan 2015). In my account settings I have ticket the setting not to delete any message to save disk space. Any idea what went wrong and how I can prevent this from happening again?