The United States has maintained the dubious distinction of being the world's No. 1 relayer of spam, but Asian countries are catching up quickly. In its quarterly "Dirty Dozen" report, released Tuesday, security firm Sophos found that four Asian nations -- Idonesia, Pakistan, Taiwan and Vietnam -- joined the list. In the third quarter of 2010, Asia was responsible for 30 percent of the world's spam, but that number now has risen to just over half. The spike appears attributable to the growing number of people coming online in the eastern part of the world. Behind the United States, South Korea, India, Russia and Brazil round out the top five contributors of unsolicited mail.