Why is there a Recovered Messages folder in Mail?

A lot of strange stuff has been going on with my older MacBook Pro (High Sierra). One of the many things is that (a couple of months ago), I brought up Mail to check my messages and all of a sudden I notice a 'Recovered Messages' folder and it shows 'Downloading 212 Sent Messages'. I just sit there staring at it trying to figure out why it's doing it and then when it's done downloading, I click on the folder, but there's nothing there, not one message. The folder is still there and now today, I bring up Mail and now there's a second 'Recovered Messages' folder there. This time there wasn't any downloading appearing, but then again, when clicking on the folder, nothing is there. I only have one account with Mail, being my @icloud address only. Is there a way to find out what triggered the creation of the folders and why, since I didn't? Is there anyway to trace iCloud activity history as I suspect that maybe someone is using it somehow? Any ideas are appreciated!