But, compare this to something like scoring against TLS encryption strength. Spammers are motivated to send as fast as possible, and strong encryption is counter-productive to that mission (increasingly so), and they can't fake it because it can be validated by a trusted relay.
Bah, spammers may be motivated by speed, but they are also opportunists and abuse the resources of others.
These days spamming is done via botnets and are almost entirely limited by the bandwidth of the node, not it's CPU time. Adding TLS shouldn't slow them down much, as it's mostly a CPU hit to do so... besides, they can always make up for it by grabbing more infected hosts.
