Barring that, perhaps something like a peer to peer network where messages are automatically encrypted and addressed to a public key could work. Those emails could then float around the p2p network until a client with that public key joined, and downloaded that message, decrypting it with their private key obviously.
I'd love it if when people sent me unencrypted email, their mail server noticed that I have a published PGP key, and encrypted the email with it before passing it on.
EDIT: Also, as for your P2P idea, IIRC there are Usenet groups where people post PGP encrypted messages for the intended recipient to pick up.
I believe every single relatively popular client supports S/MIME out of box, and a lot of them support PGP/GPG.
Most casual users just don't know/care enough about their privacy up to the moment it is severely violated..
Last time I tried sending a GPG encrypted email through Gmail on windows, there was no simple way of doing it.
I doubt there are a lot of people that would want to encrypt their messages, but still trust someone other with their keys.