The United States is maintaining its ranking as the top country in the world for distributing spam e-mails, a new report by a computer security company suggests.

The quarterly report by Sophos PLC found that between July and September, the U.S. led global spam relaying rankings at 21.6 per cent while China decreased unwanted e-mail sent from its territory almost seven percentage points since the last quarter to 13.4 per cent.

France's spam contributions rose more than a point to third place with 6.3 per cent of global output, tying South Korea, which fell more than a point to 6.3 per cent.

Sophos believes a key factor to the commanding U.S. lead in spam is a flood of virus strains that depend on the victim speaking English. More than 300 strains of the Stratio worm, also known as Stration or Warezov, trick people into unknowingly turning their computers into part of an automated spam network, or botnet.

The U.K. fell off the so-called "dirty dozen" list, falling to 13th place, while Israel has entered for the first time at No. 11, said Sophos, which has headquarters in Burlington, Mass., and Oxford, U.K.

Asia continues to lead regional rankings, taking the top spot at 34.1 per cent although its overall output fell 6.1 percentage points since the second-quarter survey.

Europe took second place with 31.9 per cent of spam, followed by North America at 24.2 per cent overall. Australasia sent the least spam in the third quarter, with just 0.5 per cent of such messages coming from the region.

In a rising trend, more than 40 per cent of all spam embeds its messages as images to increase the odds that text-based filters won't catch them. This also increases the chance of phishing attacks, which fool people into revealing sensitive information such as banking passwords by making them think they are responding to an e-mail from their bank.