As [FumbleFingers][1] 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][2] when used in E-mail.  Although the rule was not written in [the original RFC][3], 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.


  [1]: https://english.stackexchange.com/users/2637/fumblefingers
  [2]: http://en.wikipedia.org/wiki/Netiquette
  [3]: https://www.rfc-editor.org/rfc/rfc1855