Use Any Text Editor with KMail

I really like KMail for reading, organizing and searching my emails, but using its built-in editor for composing messages is a pain. I much prefer emacs. Or vim (on uneven days). Fortunately, in a good old open source manner of offering choice, KMail lets me use either:
If you'd rather use vim or emacs (or any other editor) to type and format your emails, here's how to make KMail work with your favorite editing tool.