If the recipient's email client automatically downloads the image the first time and makes it always-available to the user, wouldn't that break GhostMail's promise? While Gmail/Hotmail/etc. may not do this now, is there any technical (even an extension) reason they can't? I simply don't see how, even using a 3rd party, you can prevent somebody from holding on to the message forever. In addition, you cannot assume the content is the most important part of the message; all the email headers will exist in perpetuity, and they'll always know you sent them something (if, in the future, GhostMail wanted to do an Undo Send type feature).