As FumbleFingers already noted, the post scriptum should appear after the main body of the text, perhaps even after the signature. With that said, I do agree with FumbleFingers that the construct likely has no place in E-mail.
I would like to additionally point out that there is another use for P.S. besides adding a new thought to the correspondence: I have often seen P.S. used in E-mails to incite a new thread of conversation that is tangent to the main topic. While I think that might be a legitimate use in other forms of correspondence, it is generally considered bad netiquette when used in E-mail. Although the rule was not written in the original RFC, it is generally considered bad form to change the topic of an E-mail thread; topics should be changed by sending a separate E-mail, thus starting a new thread.
With that said, in rare cases I think the P.S. construct might be acceptable if used like a footnote, i.e., to provide some additional clarification on something from the main body of text that is likely unnecessary, and would otherwise hamper the flow of the main text.
