A year ago, computers in China were sending out 21.1 percent of all spam messages, but that number has steadily dropped over the past year, totaling just 7.5 percent in the most recent quarter, Sophos said.

During the first seven days of 2007, for example, China accounted for only 1.7 percent of spam messages, an unusually precipitous drop, said Carole Theriault, a senior security consultant with Sophos. "We saw a really significant drop in China at the beginning of this term," she said. "It's almost as if some ISPs were taken offline."

The cutoff was probably caused by two major Dec. 26 earthquakes off the coast of Taiwan, which damaged underwater data cables and disrupted Internet access in Asia, Theriault said.

But some of the credit also goes to a country-wide spam crackdown, she added. "China has been working to reduce its spam for some time now, and I think there is evidence that they have been successful."

The United States' antispam efforts have yielded more modest results in 2006. Though the United States was once the source of nearly half the world's spam, last year it totaled 23.2 percent. In this most recent quarter the number was 19.8 percent, a much more modest drop than seen in China, Theriault said.

While spam from Asia is on the decline, spammers in several European countries have stepped in to pick up the slack. In fact, 5 percent of the world's spam during the quarter could be traced back to computers connected via a single Polish ISP, Theriault said. She declined to name the company responsible, but said that it was, by far, the service provider responsible for the most spam worldwide.

Poland now accounts for 7.4 percent of worldwide spam, nearly as much as China.

Italy, France, Germany, and Spain are also included in the Sophos "Dirty Dozen" list of countries hosting top spammers.