The country has stormed into second place, accounting for 8.3 percent of the world's spam, or one in 12 junk emails seen in inboxes.
Sophos also reported that Asia and Europe are fast catching up with North America in the list of continents making the greatest contribution to the spam problem.
The US relayed far more spam than any other country between October and December 2007, largely because of the sheer number of computers in the country that have been taken over by remote hackers.
North America's 21 percent share means that more than one in five of the world's spam emails was sent through compromised US computers during the report period.
"Responsible for a third of all spam, the US and Russia are the two dirty men of the spam generation, polluting email traffic with unwanted and potentially malicious messages," said Carole Theriault, senior security consultant at Sophos.
"It is not the case that a third of the world's spammers are based in those countries, but that legions of computers are poorly defended, allowing hackers to break in and turn them into botnets for the spreading of spam and malware."
Top 12 spam-relaying countries October to December 2007:
1. US 21.3%
2. Russia 8.3%
3. China (inc. Hong Kong) 4.2%
4. Brazil 4.0%
5. South Korea 3.9%
6. Turkey 3.8%
7. Italy 3.5%
8. Poland 3.4%
9. Germany 3.2%
=10. Spain 3.1%
=10. Mexico 3.1%
12. UK 2.5%
Other 35.7%
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iTnews Executive Retreat - Security Leaders Edition
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