Via my superior reasoning and logic skills, I’m inclined to think that a P.S. was created back in the days of yore, when serfs plowed the fields, explorers were busy finding gold and peppers and people ended letters but had some extra thoughts to incorporate. Nobody wanted to waste precious paper or rewrite a ten-page letter.
With E-mail, you can revise as necessary and type anything you may have forgotten or last-minute thoughts, like the name of the girl/guy you hooked up with the previous evening. With this capability, is it still acceptable to include a P.S. in an E-mail?