Blocking spam remains a continuing challenge for Internet service providers, network administrators and government regulators. As anyone with an “Inbox” knows, unwanted commercial messages, identity theft attempts and send-money scams are as resilient as cockroaches. Worldwide spam volumes have doubled from last year, according to a filtering firm named Ironport, which says unsolicited mail accounts for more than 90 percent of Internet e-mail traffic.

Spam fighters are battling back with more sophisticated tools, including techniques to block “image spam” – a pesky new intrusion that embeds ad words in a picture to elude filters looking for telltale phrases. Image spam has surged to make up at least one-quarter of all junk e-mail, experts say. “They moved their message into our blind spot,” Paul Judge, chief technology officer of Secure Computing in San Jose, Calif., told The New York Times last week.

Filtering companies deploy a tactic called optical character recognition, which scans e-mail images to recognize letters or words. Spammers escalated the technological arms race by littering their images with speckles, polka dots and background colors that trip up automated scanners. And to elude programs that block multiple copies of the identical message, junk maile software automatically changes a few pixels in each image.

Feds frustrated: A 2003 federal law requires commercial e-mails to let recipients get off the sender’s list. Violators can be jailed, but most active spammers elude U.S. enforcers by operating from Russia, Eastern Europe and Asia.

Spam fighter says: “Imagine an archvillain who has a new thumbprint every time he puts his thumb down. They have taken away so many of the hooks we can use to look for spam. The bad guys are simply outrunning most of the technology out there today.” -- Patrick Peterson of Ironport in San Bruno, Calif.

Business e-mail blocked: Electronic newsletters, messages to customers, requested notifications and other legal contacts increasingly bounce back or are deleted automatically as filters get tighter. Overly aggressive spam blockers trash one of every five legitimate commercial e-mails, marketing executive George Bilbrey of Return Path told an industry conference last week in Park City, Utah.