You can't spoof the IP of an SMTP session, it is TCP based. Only UDP and IP can be spoofed, and that's getting harder. DNS hijacking is an attack on DNS based verifications, and DNS can be and should be secured, but you don't need dns verification to track the behaviour of IPs.

However, no anti-spam solution (other than content filters) easily deals with the botnets. They can bypass flow limiters and use the sucker's money to pay e-mail taxes and even send mail from your friends to get past whitelists. The only way to defeat that is to secure workstations from infiltration, a tall order.