How could one design a secure and “self-destructing” email?

Question

As most of you know, email is very insecure. Even with a SSL-secured connection between the client and the server that sends an email, the message itself will be in plaintex

Answer 1

The problem, as you describe it, does sound very close to the problem addressed by Vanish, and discussed at length in their paper. As you note, their first implementation was found to have a weakness, but it appears to be an implementation weakness rather than a fundamental one, and is therefore probably fixable.

Vanish is also sufficiently well-known that it's an obvious target for attack, which means that weaknesses in it have a good chance of being found, publicised, and fixed.

Your best option, therefore, is probably to wait for Vanish version 2. With security software, rolling your own is almost never a good idea, and getting something from an established academic security group is a lot safer.