P.S. is the short version of the latin phrase Post Scriptum. It is used at the end of a HAND WRITTEN letter to include something that was forgotten in the text of the letter.

Now I may be mistaken, I often am, but as far as I understand when one is writing an e-mail it is rather easy to go into the body of the e-mail and include something that one forgot. This practice would seem to eliminate the need for the P.S. part of a HAND WRITTEN letter. The nature of a HAND WRITTEN letter would dictate that it is necessary to use P.S. to include something you forgot because otherwise you would have to start over. I don't like to assume that the general public is overly intelligent but it would seem that these distinctions would be rather obvious.