Why is Mail filling up a "Recovered Messages" mailbox with what appears to be partial copies of emails I wrote? Some automatic save on drafts or what?

I discovered that for a few recent days, I received in a Recovered Messages mailbox tied to my primary email account lots of messages that appear to be ones that I was working on and that may have been partial saves of the messages that I was writing, each with a bit more or what I had written to send. Within High Sierra's Mail, is there now an autosave feature that is doing that? I had many of this yesterday, and fewer of them on a couple of days recently. I don't know what's happening, do you?