So I was trying to send a message yesterday, and a short while after I thought it sent, and while I was closing down the app, I got an error message. (which I don’t remember)
I reopened Mailspring and then I started another email draft.
The message I thought sent, is still sitting in the drafts folder on my PC, but opens the most recent draft I created, not the previous draft, which does show up right before it in the drafts list.