Like what? SpamAssassin? You would be surprised how much obvious spam gets through it in the face of the kind of spam onslaught that goes on in the real world today.
Seriously, this problem is not as easy to solve for machines as you think it is.
Seriously, this problem is not as easy to solve for machines as you think it is.
And email seems like a harder place to police, being decentralized. These guys control the gateway to the service!