The US remains the world's biggest spammer, according to security firm Sophos, which today released its quarterly report on the world's top spam-offending countries -- dubbed the "Dirty Dozen".
The US came in streaks ahead of its rivals, 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).
Australia maintained its lowly ranking on the list at number 37.
"It seems as though a major American spammer is arrested every other week at the moment, but despite these high-profile lawbreakers being put away, the US continues to relay far more spam than any other nation on the planet," said Carole Theriault, senior security consultant at Sophos, 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 US," she said.
The report also identified a growth in malicious spam containing malware, and the virtually overnight rise and fall of PDF spamming.
"The only way we're going to reduce the problem is if US 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.