Copyright © 1999–2017 FastMail Pty Ltd
There's a new option in the menu on the 'Advanced' screen when viewing messages:
"Edit as New"
This option behaves like editing a Draft message (copies all the details from the source message into a compose screen) except it will not delete the original message when you have finished.
I've wanted to write this since I joined FastMail, and here is is. Enjoy,