Reasons to insist on using a
P.S. in an e-mail?
1) Because the individual you are writing to is a newbie in the world of computing, and more comfortable with the standard routine of how snailmail letters are organized. Adding a P.S. to an email would make such a newcomer feel more at home.
2) Because the individual you are writing to is a seasoned veteran of computing and the Internet, and you know adding a P.S. to an email will probably annoy the crap out of said vet, which is the response for which you happen to be aiming.