Cause too many false positives.
* Keyed address systems (where a different email address is used per
correspondent). When an address is leaked to a spammer, the address
can be disabled and that correspondent has to aquire a new address.
Causes immense nuisance to correspondents, and the spammer can acquire the new address as soon as the pwned system is reconfigured with it.
* Feedback systems where compromised systems can be effectively
quarantined.
Users will just move to ISPs who don't kick them off the net when they get pwned.
See? It's easy to knock down every proposal with a quick dismissive comment.
mathew