I don't think signing mail in the way that S/MIME or PGP does would
solve the spam or virus problem, at least not for most of us.
I get lots of perfectly valid mail from people I don't know, and
I don't think that's unusual. It's no less valid for not being signed,
and having a signature would not do anything to convince me (or my
agent) of its validity. Even if we managed to require that all mail
be signed, spammers would create fake identities and sign those.
We might could get PGP or S/MIME widely deployed but we're nowhere
close to having the necessary infrastructure to make everyone use a
real identity in email - nor do we know how to make it trustworthy,
nor how to deal with the resulting privacy concerns.
Keith