Microsoft Corp said on Thursday that it has helped in taking down one of the biggest producers of spam email in a joint effort with federal authorities across the U.S. The company said its legal action against the operator of the Rustock 'botnet' led to raids across the country on Thursday, which effectively shut the network down. Tech security bloggers who follow the activities of botnets around the world noticed a sharp dip in spam email coming from the Rustock network, which some say is responsible for almost half of the spam appearing in inboxes. Microsoft said that it worked with the U.S. Marshals Service to raid hosting providers in seven U.S. cities early on Thursday, including Denver, Dallas, Chicago, Seattle and Columbus. "With help from the upstream providers, we successfully severed the IP addresses that controlled the botnet, cutting off communication and disabling it," Richard Boscovich, senior attorney at Microsoft's Digital Crimes Unit in a blog post on Microsoft's site, said in a statement. "This case and this operation are ongoing and our investigators are now inspecting the evidence gathered from the seizures to learn what we can about the botnet's operations." The world's largest software company said it believes there may be a million computers infected with Rustock malware, sending out as many as 30 billion spam emails per day. The Rustock network was responsible for many of the fake lottery scam and pharmaceutical offers that email users have found. Rustock has been said to be capable of sending as many as 30 billion emails per day. Microsoft said it was working with Internet service providers and response teams around the world to help users get Rustock malware off their machines. Boscovich said that many of the emails sent by Rustock advertised counterfeit or unapproved knock-off versions of drugs like Viagra. He said that Rustock was knocked offline on Wednesday when the connection was severed between infected computers and the machines used to give them orders.