The US remained at the top of the chart of spam-relaying nations during the second quarter of 2006, accounting for 23.2 per cent of the world's unsolicited email.
The closest rivals to the US are China and South Korea, according to figures from IT security firm Sophos, although both of these nations have managed to reduce their statistics since the first quarter of 2006.
But the US has failed to reduce its spam problem for the first time in more than two years.
The vast majority of the spam tracked by Sophos was relayed by botnets of 'zombie' computers hijacked by Trojan horses, worms and viruses under the control of hackers.
"Since the introduction of the US Can-Spam legislation in 2004, we have seen a regular quarter-on-quarter drop in the proportion of spam coming from the US. Until now, that is," said Graham Cluley, senior technology consultant at Sophos.
"Given the number of arrests, and the huge fines dished out to guilty spammers, it is hard to criticise the US for failing to take action.
"Perhaps the reality is that the statistics cannot be reduced any further unless US home users take action to secure their computers and put a halt to the zombie PC problem."
Sophos found that Asia accounts for more spam than any other continent, but that spam relaying in Europe continues to become more prevalent.
While 25 per cent of the world's spam was sent out from European countries in the first quarter of 2006, the figure has now reached 27.1 per cent. Europe has now overtaken North America as a spreader of spam.
Even though Russia does not feature in the dirty dozen of spam relaying countries, Sophos claims to have uncovered evidence that Russian spammers may be controlling "vast networks" of zombie PCs.
The firm recently discovered a Russian spamming price list, which showed that $500 would purchase email distribution to 10 million Russian email addresses. It also offered distribution to one million addresses in any country for just $50.
One key development in 2006 has been the increase in spam containing embedded images, which has risen sharply from 18.2 per cent in January to 35.9 per cent in June.
By using images instead of text, messages are able to avoid detection by some anti-spam filters that rely on the analysis of textual spam content.
Sophos estimates that 15 per cent of all spam emails are now 'pump-and-dump' scams, compared to just 0.8 per cent in January 2005.
These scams are email campaigns designed to boost the value of a company's stock in order for spammers to make a quick profit. Many of these spam messages contain images rather than traditional text.
"It is worrying to see so many pump-and-dump emails, often with embedded graphics, being spammed out to the general public," said Cluley.
"The people that act on these emails are not skilled investors, and do not realise that purchasing the shares is likely to reap no reward, benefiting only the spammers while creating a financial rollercoaster for the organisation in question."
The top 12 spam relaying countries April to June 2006 are as follows:
1. United States 23.2%
2. China (inc. Hong Kong) 20.0%
3. S Korea 7.5%
4. France 5.2%
5. Spain 4.8%
6. Poland 3.6%
7. Brazil 3.1%
8. Italy 3.0% new entry
9. Germany 2.5%
10. United Kingdom 1.8%
11. Taiwan 1.7%
12. Japan 1.6%
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