My wife wanted to forward an email from Mail to a friend. The email had a video attachment. She selected Forward, selected the unwanted part of the original email and pressed Delete. The entire forward window plus the original email (in her inbox) vanished. By good fortune, I thought to look in On My Mac > Recovered Messages. From there, it was easy to move it to her inbox (and the original message was gone, although the attachment was there) and send it to the intended recipient.
I am at a loss to explain how this happened. Any ideas?