Report: U.S. tops list of spam-offending countries

The U.S. came in well ahead of its rivals, according to the report, being responsible for 28.4 percent of all spam. South Korea was second (5.2 percent), followed by China (4.9 percent), Russia (4.4 percent) and Brazil (3.7 percent).

"It seems as though a major American spammer is arrested every other week at the moment but, despite these high-profile law-breakers being put away, the U.S. continues to relay far more spam than any other nation on the planet," Carole Theriault, senior security consultant at Sophos, said in a statement.

"This level of activity can't be attributed solely to the slick operations of a few cash-hungry criminals. The problem is there are thousands of spammers using many thousands of compromised zombie computers in the U.S.," Theriault said.

The report also identified a growth in spam that contains malicious software, and the virtually overnight rise and fall of PDF spamming.

"The only way we're going to reduce the problem is if U.S. authorities invest a lot more in educating computer users of the dangers, while ensuring ISPs step up their monitoring efforts to identify these compromised machines as early as possible," added Theriault.

Marcus Browne of ZDNet Australia reported from Sydney.