A postscriptum was added to a handwritten letter to avoid having to copy all of it only to add in an afterthought. When you write digitally and have the option to easily rewrite any part of your letter before you email it, a postscriptum is completely out of place.
An email, especially to a person of consequence to your carreer, should be a carefully thought out and crafted piece of writing, so take the time and make the effort to work in your afterthought into the body of your email. Even if it is completely unrelated to the rest of your email, there is still no reason why you could not open another topic and insert it before the complimentary close.
P.S. Joan just told me that she will be in Paris tomorrow and could meet you on short notice.
Joan will be in Paris tomorrow, and if you have time for a meeting on such short notice, let her know.
If you must add a postscript, and you give your address as a signature at the end of your email, add the postscript immediatly after the complimentary close, and before the signature. The postscript is part of the "letter", the signature is not.
Many signatures are separated from the email by a dashed line or other elements, and adding the postscript after the signature will make it appear as part of that signature (and possibly overlooked).
Do not do this:
25, Your Street
Your City, NN 12345
P.S. Stuff that will seem as if you add it to every email and will be ignored.