Security solution provider Marshal said that the Srizbi bot now sends more spam per day than all other botnets combined.

"Srizbi is the single greatest spam threat we have ever seen," said Bradley Anstis, vice president of Products at Marshal. "At its peak, the highly publicized Storm botnet only accounted for 20 percent of \[all\] spam."

Marshal said that it estimates the Srizbi botnet to be comprised of over 300,000 PCs, and that the botnet sends over 60 billion spam messages per day. Based on Marshal's data Srizbi sends between 45 to 55 percent of all spam detected by the company's spam traps. The next closest botnets in terms of spam volume are Mega-D and Rustock, which are responsible for less than 20 percent all spam detected by Marshal. The company said that its researchers have seen Rustock-infected computers send as many as 25,000 spam messages an hour.

"While other botnets such as Rustock might be smaller in terms of the number of compromised computers under their control they can be very efficient at sending spam," Anstis added.

Marshal hopes to see Microsoft go after the Srizbi botnet the way it went after the Storm botnet. Microsoft used its Malicious Software Removal Tool to help eradicate the malware, which in turn reduced Storm's effectiveness. Storm is now responsible for less than 2 percent of all spam according to Marshal's findings.