30 million spam emails a day.

That is how many that the Rustock email spamming network was putting out until it suddenly went dark, and Internet security experts didn’t have a clue as to why.

Well it turns out that Microsoft, along with the U.S. Marshal Service, was responsible. It was all a part of a co-ordinated sting in seven U.S. cities and in Holland. (Microsoft’s involvement stems from the fact that t many of the emails sent out through the botnet abused its “Microsoft” and “Hotmail” trademarks.)

Using a method similar to the one that helped Microsoft shut down the Waledac botnet back in February 2010, the company went to court. Unlike the the Waledac case however, Microsoft this time used a trademark-infringement lawsuit to get a U.S. District Court judge in Seattle to get an order to seize the implicated equipment, as allowed under the Lanham Act.

Such take-down operations require high coordination and secrecy, John Bambeneck, a member of the SANS global Internet monitoring service, told The Wall Street Journal.

“They all had to have been taken down simultaneously or they would have noticed and been able to react,” he told the WSJ’s Digits blog.

[Cross-posted at Winextra]