Microsoft is suing a serial spammer from Connecticut who found a way round the spam filters at the software giant's Hotmail online email service.
Boris Mizhen is accused of circumventing the company's spam filters by creating millions of new email accounts and using them to classify his messages as "not spam".
The law suit, filed last week in the federal district court in Seattle, names a number of other companies, including three online advertising firms that Mizhen allegedly controls: Media Network Inc, New Age Opt-In Inc and Permission Inc.
Microsoft accuses the company of fooling Hotmail's spam filters by moving up to 200,000 of their own messages a day, which had been classified as spam, out of the 'junk' folder and back into in-boxes.
One of Mizhen's associates used evidence of the messages being moved to convince Microsoft to reclassify the companies' messages.
According to Microsoft's complaint, "The defendants' deceptive conduct allowed them to circumvent Hotmail spam filters and to continue to disseminate a vast quantity of spam email messages to legitimate Hotmail users."
The company alleges Mizhen and co-defendants have violated a number of laws, including the 2003 federal 'Can-Spam' Act.
Mizhen has a long history as a spammer. For a number of years he was listed on the ROKSO list by anti-spam organisation Spamhaus. In 2003, he settled a previous case with Microsoft by agreeing to pay $2 million and to refrain from spamming Hotmail users.