Spam sender

Graham Cluley, Sophos senior technology consultant, said that in 2004 more than half of all the spam in the world was coming from the US.

This has dropped, he said, because the US was making efforts to find and fine prolific spammers in its borders.

"We're seeing spammers being successfully brought to justice in the US, but it's important to remember that they can be based anywhere in the world," said Mr Cluley.

The Sophos statistics show the country from which spam is relayed rather than where it originated.

More than 70% of the 13.2 billion spam messages sent every day are now believed to be relayed via hijacked home computers of unsuspecting net users.

Many of the malicious hackers controlling these herds of so-called zombie computers rent them out to spammers who want to hide the origins of their unwanted e-mails.

In a typical month half of the most active computer viruses are those tailored to find vulnerable home computers and turn them into zombies.

"It's imperative that computer users worldwide put better defences in place to prevent their computers from being converted into spam-spewing zombies," said Mr Cluley.

He added that although spam from the US is in decline, the slack is being taken up by other nations. According to Sophos, the spam relaying figures of Poland, Spain and Germany all rose in the last six months.

Information about where spam is being sent from was collected via the network of junk mail traps that security Sophos maintains around the world.