Many people agree that crypto is part of the long-term picture of email.
S/MIME was originally designed as end-user to end-user infrastructure.
Recently a domain-to-domain version has been announced:
http://www.opengroup.org/messaging/sm/smgdev/doc.tpl?CALLER=index.tpl&gdid=4657
But it still looks heavyweight compared to Yahoo DomainKeys.
Wouldn't it be great if S/MIME could, instead of requiring a
cumbersome and O(n^2) initial key exchange between domains, make use
of opportunistic "oh, look, and here's the cert in DNS"?
Oh, and instead of "cert" read "self-signed public key".
Then maybe we'd get the benefits of the Yahoo DK infrastructure but
leverage the existing S/MIME base.