Empty E-mail folder

Since the dawn of time I've been saving my archived POP e-mail in "Archives/YEAR/IN and OUT" folders. I just tried to read an e-mail in my "2018 IN" folder. In Mail v 13.4 could see the preview of an e-mail, but could not open the e-mail. After several attempts I restored the folder, which apparently emptied the contents of the folder, because now it's empty of all e-mail.

I did a search and created a smart mailbox for "2018" and only found sent e-mails, no IN emails.

I (hopefully) have a 2 day old Time Machine backup, and maybe a cloud backup of my Home Library, but I can't find where the "Archives" and "2018 IN" folder would be in order to only back up that folder. I see plenty of folders in my "/Users//Library/Mail/V7/" including "IN" but there is no "Archives" folder that I can find in all those subdirectories. Spotlight doesn't find it either. I'd rather not back up the entire mail folder because I don't know what repercussions that would have.

Any ideas?

Thanks!

Catalina v10.15.7