In March, Microsoft was the driving force behind shutting down one of the most prolific spam operations on the Internet — a botnet called Rustock. The sting put an end to the more than 13 billion spam messages Rustock was generating daily, which represents a major victory for legitimate marketers and businesses on the Web. One battle, however, does not win the war.

Online security firm MessageLabs reports that despite temporarily eliminating a third of all spam in one fell swoop, another botnet called Bagle is picking up where Rustock left off. Bagle is now responsible for nearly 8.5 billion spam messages daily, mostly in the pharma industry, and is considered by MessageLabs to be “more consistent” than Rustock.