The scourge of spam e-mail will likely never go away, but Microsoft says new data shows that a few targeted anti-botnet operations can reduce malicious e-mail volume by tens of billions of messages per month.
In July 2010, 89.2 billion spam messages were blocked by Microsoft’s Forefront Online Protection for Exchange service, which is used by thousands of enterprise customers. By June 2011, that monthly total was down to 25 billion. Microsoft, in the latest bi-annual Security Intelligence Report (PDF) covering the period ending in June, attributes the drop primarily to the “takedowns of two major botnets: Cutwail, which was shut down in August 2010, and Rustock, which was shut down in March 2011 following a period of dormancy that began in January.”
Some good overall statistics here.