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 !
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 ...
Michael -- IERenderer's Homepage PGP Key ID (RSA 2048): 0xC45D831B S/MIME Fingerprint: 94C6B471 0C623088 A5B27701 742B8666 3B7E657C