So I read about a service called BigString.com where users can send emails that can be recalled / edited / expired / non-forwarable, w/o any cooperation of the recipients mail user agent (MUA).
It turns out that their system is based on sending people a HTML email that contains a IMG link to a picture of the email. The MUA loads this picture from the BigString.com server. Since the image is on their server they control whether or not it is available. Of course, their claims of recallable / expirable email falls flat as soon as the user does a "Save-As" of the picture.