Every once in a while an email in my Thunderbird inbox disappears into a mysterious folder called "2019", a folder that is not in my folder list. What happens is that I read an email, close it, then, when wanting to check it again later, find that it is gone. Not in the inbox, not in the trash. If I use the search option, I find that the email is still there in a location listed as "2019". I can open the email it from the search facility but haven't found a way to open folder 2019.
Folder 2019 is not in the folder list. It isn't a subfolder. I'd really like to know what else has disappeared into that mysterious black hole. I'm positive there are some important emails lurking there. I would very much like to refile them in the appropriate subject folders so that I can find them when I need them.
Any suggestions as to how to locate this mysterious folder are appreciated. Thank you!