Spammers are co-opting the top-level domains (TLDs) of obscure island nations as a new tactic to avoid spam filters, according to McAfee.

In the past, spammers have ordinarily used sites with well-known TLDs such as .com or .info to advertise their wares. But McAfee's researchers began to notice a steep rise in the number of junk emails using .st domain names, .st being the TLD for Sao Tome and Principe, islands off Africa's west coast.

Digging further, they found that Sao Tome and Principe aren't alone, with domains turning up from the Isle of Man, Tokelau in the South Pacific, Tonga, Tuvalu and others.

"Some of these islands have dozens of spammed domains per square mile," said McAfee senior development manager Guy Roberts, in a statement.

Indeed, Sao Tome and Principe, with 1,001 square kilometers of land area, are the biggest of the islands to have turned up in McAfee's survey. The other TLDs favoured by spammers, in descending order by size, are Tonga (.to), the Isle of Man (.im), American Samoa (.as), Tuvalu (.tv), Cocos (Keeling) Islands (.cc) and Tokelau (.tk), the smallest at 10 square kilometres.

The technique is intended to confuse spam filters, which examine in-message links as one way of detecting junk email.