In March 2011, a Microsoft-led team targeted and decapitated the Rustock botnet, and a dramatic decrease in spam traffic was noticed almost immediately. It turns out that a full year later, spammers have not been able to fill the gaping hole left by Rustock's absence.

Just before the Rustock takedown, "spam levels were around the 150 billion mark daily," security vendor Commtouch said in a new analysis. "Spam levels dropped immediately after that takedown and have continued to decrease ever since. In the first quarter of 2012, an average of 94 billion spam emails were sent per day… There is no sign of a return to pre-Rustock spam levels."

Rustock was responsible for sending 30 billion spam e-mails a day, and thus its takedown alone can't account for the entire drop in spam volume. Commtouch said the sustained improvement was a combination of multiple botnet takedowns, as well as "increased prosecution of spammers and the source industries such as fake pharmaceuticals and replicas."

Spam accounted for 75 percent of all e-mails sent in the first three months of 2012, according to Commtouch's "April 2012—Internet Threats Trend Report." Commtouch said it is "tempting" to conclude that a decade-long growth in spam has been permanently reversed, but the signs are not all good. Commtouch estimates that 270,000 zombie computers were activated each day for the purposes of sending spam in the first quarter of 2012, up from 209,000 in the last quarter of 2011. There had been a drop in November because of the "Esthost" botnet takedown, but "spammers have worked to source new zombies since the start of 2012," Commtouch said.

Commtouch's estimates are based on traffic going through its GlobalView Cloud service, which handles more than 10 billion transactions each day, including "URL and spam queries from millions of endpoints." The data does not include internal corporate traffic.