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

If the recipient knows that the message might become unreadable later and they find the message valuable their intention will be to preserve it, so they will try to subvert the protection.

Once someone has seen the message unencrypted - which means in any perceivable form - either as text or as screen image - they can store it somehow and do whatever they want. All the measures with keys and so one only make dealing with the message inconvenient, but don't prevent extracting the text.

One of the ways could be to use self-destructing hardware as in Mission Impossible - the hardware would display the message and then destroy it, but as you can see it is inconvenient as well - the recipient would need to understand the message from viewing it only once which is not always possible.

So given the fact that the recipient might be interested in subverting the protection and the protection can be subverted the whole idea will likely not work as intended but surely will make dealing with messages less convenient.