Several related articles in re the energy consumption of spam, and the
economic/energy returns earned by countering it effectively:
http://www.crn.com/security/216500943;jsessionid=PIRZRMECSH3WOQSNDLPCKH0CJUNN2JVN
http://news.cnet.com/8301-1023_3-10220363-93.html
http://www.newsfactor.com/news/Blocking-Spam-Would-Cut-Pollution/story.xhtml?story_id=03300316RKPR
Hardly as scientific, but Exim with tightened rDNS filtering et al has
allowed us to drop from Core-2 Duo 3 GHz to VIA C3/C7 at a mere fraction
of former power consumption...and cost.
YMMV, but what works here is to reject it early, not analyze and 'play'
with it.
Bill Hacker