Junk e-mail continued to hit mailboxes around the world Thursday, despite the arrest a day earlier of a man described as one of the world's most prolific spammers.

Even if Robert Alan Soloway is ultimately convicted and his operations shuttered, spam experts say dozens are in line to fill the void.

"In the short term, the effect it's going to have is more symbolic more than anything else," said John Levine, co-author of "Fighting Spam for Dummies." "Soloway is a large spammer, but hardly the only large spammer."

• Click here to read the federal indictment of Robert Alan Soloway (pdf).

Levine said Soloway was a good target because he operates in the United States and has taken few steps to cover his tracks.

Soloway, 27, was once on a top 10 list of spammers kept by The Spamhaus Project, an international anti-spam organization. Others have since topped him, mostly based in Russia and other countries out of reach of U.S. or European law.

But Soloway remains on a Spamhaus list of about 135 spammers deemed responsible for as much as 80 percent of all junk e-mail, and one Spamhaus official considers him in the top 20.

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"Most of the Russian gangs seem to have a lot more freshly hijacked computers and are able to deliver much more spam into people's inboxes," said Vincent Hanna, a European investigator for Spamhaus. "The stuff that Robert Soloway had under this control, let's call it second-grade."

Soloway was arrested Wednesday on charges of mail fraud, wire fraud, e-mail fraud, aggravated identity theft and money laundering.

Prosecutors say Soloway has sent millions of junk e-mails since 2003 and continued even after Microsoft Corp. (MSFT) won a $7 million civil judgment against him in 2005 and the operator of a small Internet service provider in Oklahoma won a $10 million judgment.

Soloway could face decades in prison, though prosecutors said they have not calculated what sentence range he might face.

Like most spammers these days, authorities say, Soloway sent out unsolicited bulk e-mails using networks of compromised computers called "zombies."

These are generally home computers whose owners typically have no idea that their machines have been infected with viruses or other malicious programs; service providers can't easily block messages from zombies because they are mixed in with legitimate messages.

What set Soloway apart was his focus on spam designed to sell tools and services for companies and organizations to send their own junk e-mail, said Patrick Peterson, vice president of technology at anti-spam vendor IronPort Systems Inc.

Other types of spam were largely unaffected by the arrest. One Gmail account collected messages Thursday promising deals on Viagra and other medicines, while an AOL account drew an offer for two large, mouthwatering pizzas.

IronPort said it saw no notable drop in spam volume, with 70 billion messages in a 24-hour period, unchanged from two weeks earlier. The company said spam has doubled from about 36 billion a day last May.

Anne Mitchell, who runs the anti-spam consultancy Institute for Spam and Internet Public Policy, said an individual user who happens to be on Soloway's list might see a big drop, but the collective impact is negligible.

"It's not that different from the Mafia," she said. "Many times the feds grab a high-ranking don, but the Mafia didn't go away. Someone's going to step up and fill his void."

Nonetheless, anti-spam experts lauded the arrest, calling it an encouraging sign that authorities are taking spam seriously. Compared with civil lawsuits that have led to multimillion-dollar judgments, prosecutions have been rare.

"Criminal prosecutions are absolutely necessary," said Richi Jennings, lead analyst for e-mail security with Ferris Research. "It adds a whole new level of fear to the lives of these spammers."

Microsoft spokesman Lou Gellos said the arrest not only removes Soloway from spamming "but it throws caution to others that are doing it as well. There's hope that this causes a ripple, if not a wave."

At Wednesday's news conference in Seattle announcing the arrest, U.S. Attorney Jeff Sullivan acknowledged that "others sometimes take their place, but we want it to be a deterrent."