[quote user="davereid"]
Currently you can easily encrypt a message - as long as you have exchanged the key with the recipient.
Public Key systems overcome this, but they are so hard to use, you need to be a expert just to create your public and private keys !

[/quote]

S/MIME kind of does it this way: It (by default) includes the public key (as part of the "certifcate" = signature) so send a signed message to someone whom you'd want to encrypt emails to you. If you're interested watch the Encryption forum: Although I don't like the upside-down concept of S/MIME & certificates too much it's like with all "standards" pushed by M$: We have to support them at least in some basic way ...

Currently you can easily encrypt a message - as long as you have exchanged the key with the recipient.
Public Key systems overcome this, but they are so hard to use, you need to be a expert just to create your public and private keys !
But.. what if it was all done by your email client for you.
So.. for example, I have never written to you before, but I would like to send you an email.
I write it in Pegasus, put in your address and send it. But Pegasus doesn't send it.
Pegasus automatically sends a request to you, for your public key.
Your copy of Pegasus receives the request, and automatically sends you the public key.
Then we are talking - all with no user intervention.
If the recipient isnt using Pegasus, they will just receive the email requesting their public key - they can send it if they have one, or just reply "sorry, I don't care who reads my emails just send it plaintext"

You could also use the PGP Key-Servers for this scenario.

I think it's not the business of e-mail programs to exchange the keys.