Researchers at security software company Sophos found that 42 percent of all spam sent this year came from the United States, based on a scan by its researchers of a global network of honey pots--computers designed to attract spam e-mails and viruses.
Sophos said this is evidence that America's antispam legislation simply isn't working.
"When we released the first report back in February, the U.S. had the excuse that the Can-Spam Act had been in existence for only three months," said Graham Cluley, senior technology consultant for Sophos, last Friday.
South Korea and China came in second and third place, respectively, but together they sent just half as much spam as the United States.
Sophos warned that many spammers are using hacked PCs with broadband connections to send out their spam. This could explain South Korea's position near the top of the list, as it leads the world for broadband penetration.
"Spammers are motivated by one thing--quick, easy money," Cluley added. "There are plenty of spammers who have taken their money-making schemes to the extreme by hacking into innocent third-party computers in an effort to do their dirty work."
"Many of the computers sending out spam are most likely to have had their broadband internet connections exploited by remote hackers. Zombie computers--PCs that have been compromised by hackers or virus writers--are sending out over 40 percent of the world's spam, and many users who fall victim are unaware," he said.
Dan Ilett of ZDNet UK reported from London.