I apologize if these issues came up before I joined the list but...
a) All existing e-mail signature tools sign the bodies of messages,
not the headers. They have no way to authenticate the headers unless
they duplicate them in the body and your tool compares them. So
they don't do a lot to prevent forgeries of headers, subject lines
etc. which are pretty important in this case.
b) X.509 uses identity certificates rather than attribute certs, and
the certs are HUGE and people would get quite annoyed attaching 5K
to each e-mail.
c) Identity certs are evil, did I mention that? :-)
d) Microsoft outlook supports S/MIME and x.509 but in a terrible way.
If you turn it on, you get queried with dialog boxes about access
to your private key with every email you send. (plus every email
you receive encrypted.)
e) You can declare you demand signed mail, but what good will that
do you during the early period when nobody is signing? You have
to have some other avenue for them. And why should people sign if
the other avenue exists? It's a tradeoff on how hard you make the
avenue. Make it too hard and lose mail. Make it too easy and
nobody feels the need to get all new email software and a cert.
Frankly, ask me to do anything particuarly hard just to send you
a mail, and I'm going to say "fuck you" unless I really had a
strong interest in sending that mail. If I was sending it to help
you, because of request on your web page or in a posting, and I get
more than the simplest of challenges, I will just discard the mail.
f) You still need systems to allow anonymous mail (just not in bulk)
and not have everybody by default refusing it because it might be
spam.
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