Somebody must already have thought of this. It is such a simple concept. And devious.
When replying to an email insert really idiotic spelling mistakes into the quoted text from the original email. Imagine the cumulative psychological effect. "Jeez, did I write that. How embarassing." Immediately you have the guy on the back foot - useful for delicate negotiations.
Of course, since ninety percent of email users can't spell anyway, it is quite possible that nobody will notice. I guess the next level would be to completely bork their original text (swedish chef style). Alternatively just start pointing out their(your) spelling mistakes.
You could also insert sentences into the quoted text - say a random quote from Ralph Wiggum (Simpsons), or a suggestion that you really deserve a massive pay rise (if replying to The Boss).