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Robert Soloway, 28, had already been found guilty of spam charges in several civil cases -- Microsoft won a $7.8 million judgment against him in 2005 -- but had avoided paying fines in those cases. The criminal charges to which he pleaded guilty on Friday followed his arrest in 2007 by the U.S. Justice Department.

He was arrested on criminal charges brought by the U.S. Department of Justice in May 2007.

In a 2005 discussion group post, Soloway bragged, "I've been sued for hundreds of millions of dollars and have had my business running for over 10 years without ever paying a dime regardless to the outcome of any lawsuits."

That year, Soloway raked in more than $300,000 from his spam operations, according to his plea agreement.

Soloway has avoided fines in the past, but this time around he may not be so lucky. In addition to the jail time he now faces, he has also agreed to discuss his financial assets while being monitored by a lie detector.

While there have been hundreds of spam prosecutions in the United States, it is extremely rare for spammers to face criminal charges, and those involved in the matter say that Soloway's case could serve as a deterrent to other spammers.

In an interview last month, Microsoft senior attorney Aaron Kornblum said he thought the prosecution would make other spammers think twice. "There have not been a large number of criminal CAN-SPAM prosecutions in the U.S.," he said. "This is significant."

Soloway is set to be sentenced on June 20. The prosecution had been seeking $700,000 in damages when Soloway was first charged nearly a year ago.