A notorious spammer is facing up to three years in prison after pleading guilty to charges related to sending millions of unwanted email messages after breaching Facebook's computer network.

Sanford Wallace, who called himself the Spam King starting in the late '90s, pleaded guilty this week to one count of fraud and related activity in connection with email and one count of criminal contempt.

Between November 2008 and March 2009, Wallace fraudulently obtained Facebook users' login credentials in order to send spam email, the U.S. Department of Justice said, citing his plea agreement. Wallace pled guilty on Monday in U.S. District Court for the Northern District of California.

During three intrusions into Facebook's computer network, Wallace accessed about 500,000 user accounts and sent more than 27 million email messages through the company's servers, the DOJ said. Wallace, 47, of Las Vegas, targeted Facebook users with hundreds of thousands of the spam emails, the agency said.

As part of his plea agreement, Wallace acknowledged violating an order by Judge Jeremy Fogel related to a civil case filed by Facebook against him. Three separate times in 2009, Fogel, of the Northern District of Calfornia, ordered Wallace not to access Facebook's computer network.

Wallace was indicted by a grand jury in July 2011. He faced 11 criminal counts, including six counts of fraud and related activity in connection with email. He is scheduled to be sentenced on Dec. 7 in San Jose.