Spam volume has significantly declined since hosting company McColo went down on Tuesday night. The site went down after four months of investigation by security experts and an investigative piece by Washington Post reporter Brian Krebs, which claimed that 75% of all spam was being sent via the hosting service.
From CNET:
"Jose Nazario of Arbor Networks, a company that monitors botnet activity, speculated that McColo vanished at around 9 a.m. Eastern time on November 10. Botnets are frequently used to relay spam, and McColo may have hosted some of the command and control servers necessary to coordinate spam campaigns"
