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?

Speaking from my experience in Tech Support for Microsoft, in situations like you describe it is almost impossible to speculate on what happened unless you can directly observe the process while it is in progress. All I can offer is a process that mitigates the risk of loss. Assuming your wife was using the Apple Mail app rather than deleting the portion of the message she didn't want it is safer — and IMO easier — to…

1. select the portion of the original you want in the reply or forwarded message
2. then click on the appropriate send or reply icon
3. add her text, forwarding addresses, etc. in the draft window that appears.
4. When editing is completed click on Send

If there is an error the original message has not been touched, so it is easy to start over from scratch.

AFIK, clicking "Forward" does not change the original email but copies it to a different window for editing prior to sending. At least, that seems to be the way it has always worked for me. The forwarded message ends up in the Sent mailbox while the original stays in Inbox. And yes, she is using Apple Mail.

Am I missing something?

Agreed, but if she has selected the text in the original mail before clicking forward, only the selected text will be in the edit window.

As both my wife and I suspected, the cause was PEBKAC. She started to edit an email BEFORE clicking Forward. She ran into a similar difficulty today but I was able to see what was happening. She is great with animals, having tamed two feral cats and two feral snakes. So, she thinks like a cat and thinks like a snake, but she doesn't think like a computer. I won't ask what she thinks about me! (Quip: Never feel guilty because you have thought ill of your wife. She has thought much worse of you.)