Authorities say it was behind billions of unsolicited e-mail messages.
Microsoft and New York Attorney General Eliot Spitzer are suing what they describe as a "spam" ring that they say was responsible for sending the messages.
The lawsuits target a man identified as Scott Richter, who's alleged to be one of the world's most prolific senders of spam.
The suits accuse Richter and what they call "accomplices" of sending illegal spam in 35 countries -- and disguising their work to keep people from tracing the messages.
Richter tells The New York Times the lawsuits are baseless.