
In a study of spam received over the second half of 2006, Symantec found that roughly 45 percent of the messages were sent from computers within the US. The volume of spam coming from the US was more than seven times as much as the second-largest purveyor, China.
"This is not to say that the spammers themselves are American," Symantec security researcher Nick Sullivan said on the company's security response blog.
"The purveyors of illicit pharmaceuticals, gurus of pink sheet penny stocks, and so-called representatives of 'your bank' may very well be from China, Russia, Brazil, and other countries, but the spam itself is sent mostly through American computers.
The most likely cause of the high rate of stateside spam, said Sullivan, is that spammers prefer to use American ISPs and free e-mail services (such as Hotmail or Yahoo Mail) to spread junk mail.
He also suggested that US botnets are specializing in sending spam because of the proliferation of broadband internet accounts that make it easier to send large volumes of spam messages.
He admitted however that the last theory has some flaws. Some regions in Western and Central Europe have much higher broadband penetrations than the US, yet they account for a relatively small percentage of spam.