How can you destruct already delivered email?

I'm going to guess its HTML emails that use javascript to download a decryption key. The js could hash it's own functions to avoid tampering. Rudimentary cut-and-paste protection could be enabled (don't know if the MUA would honour it though). Remove the key from the URL when you want the email to self-destruct (I'm thinking htaccess going to a db so you've set the date in the db beyond which the URL redirects to a key which causes the display of "sorry this email has expired" rather than the plaintext. Add a print-style sheet to blank out an attempt to print it.

Wouldn't work against screenshots and would be reasonably easy to crack but for the "man in the street" I can see it working?

I'm sure there is a far better more devious way of doing this.