As I've said before: prime numbers are cheap. Anyone can get a
certificate. If we do this, we'll just get certified spam.
right, spammers will continue to get throwaway identities just
as they do now. so whether a message is signed won't tell you
whether it is spam.
however if you know that the message comes from a trusted source,
you might give it preferential treatment. for instance, you might
allow it to be delivered immediately. if you aren't sure about
the message source, you might refuse to accept the message for
some period of time - a few hours or a few days.