to reproduce, use win10x64, outlook 2016, and greenshot 1.2.10 build 6 (64bit).
open outlook. select any email, right click email and open "move to folder > other folder" which will open a choose folder dialog. While this dialog is open, take screenshot and choose send to email.
it will complain and not create the email, but after you close the outlook "move to" dialog, the screenshot appears lost.
It would be nice to re-prompt for the destination if it fails to open the email screen and attach the screenshot.
..or save it to the default location so it is not lost.
win10x64, MS outlook 2016, and greenshot 1.2.10 build 6 (64bit).