US efforts to stop the nation serving as the world's top spam relaying nation are stalling.

For the first time in more than two years, the US has failed to rein in the hordes of botnet computers that made it the source of 23.2 per cent of the world's spam in Q2 2006.

Its closest rivals are China and South Korea, although both of these countries managed to reduce the amount of spam they were responsible for between the first and second quarters of 2006, according to an analysis of the top 12 spam relaying countries by UK-based net security firm Sophos.

Sophos says the prosecution of spammers under the US's CAN-SPAM Act needs to be accompanied by improved home security if hopes of reducing spam are to be realised.

During Q2 2006, Europe overtook North America as a spreader of spam, with an increase of 2.1 percentage points over the quarter leaving it responsible for 27.1 per cent of global spam in Q2 2006.

Although Russia does not feature in the dirty dozen of spam relaying countries, Sophos reckons Russian spammers control vast networks of compromised (zombie) PCs in other countries.

Sophos notes an increase in spam containing embedded images, up from 18.2 per cent in January to 35.9 per cent in June, a ruse designed to fool some anti-spam filters that rely on the analysis of textual spam.

There's also been a growth in spam messages designed to inflate the value of company stock (so called pump and dump spam) which currently accounts for 15 per cent of spam messages compared to just 0.8 per cent in January 2005. ®

Top 12 spam relaying countries (Q2 2006), according to Sophos

1. United States (23.2 per cent)
2. China (20.0 per cent)
3. South Korea (7.5 per cent)
4. France (5.2 per cent)
5. Spain (4.8 per cent)
6. Poland (3.6 per cent)
7. Brazil (3.1 per cent)
8. Italy (3.0 per cent)
9. Germany (2.5 per cent)
10. United Kingdom (1.8 per cent)
11. Taiwan (1.7 per cent)
12. Japan (1.6 per cent)