The U.S. is the spam leader across the world, responsible for one out of every five junk messages sent, according to a report out today from Sophos.

The security vendor's fourth-quarter "Dirty Dozen" report of spam-relaying countries found that the United States upped its percentage of global spam from the third quarter and now accounts for 18.83 percent of all junk e-mails.

That percentage is almost three times higher than second-place India, which is responsible for deploying 6.88 percent of all spam across the globe, according to Sophos. Other countries named on the Dirty Dozen list include Brazil, Russia, the U.K., and France.

Beyond producing the sheer volume of spam per country, the bad guys are finding more malicious ways to spread their payloads.

Though junk mail still relies on typical subjects, such as ads for pharmaceuticals, more spam is being geared toward sending malware and stealing user names, passwords, and other personal information, says Sophos. Another form of attack on the rise is spear-phishing, which sends out e-mails targeted to specific organizations or people, usually in an attempt to gain confidential information.

Following the takedown of the Bredolab network late last year, spam levels started to take a dip. Spam also has been in a slump since Christmas as the Rustock spambot, responsible for generating a lot of the world's junk mail, seemed to take some time off for the holidays.

But Sophos believes the malware writers have actually turned their attention to using Rustock for other tasks, such as installing pop-up ads and trying to get people to buy phony antivirus software. And as of yesterday, the security vendor reports seeing spam levels spiking up yet again. As always, Sophos cautions people to be wary of e-mail attachments and unknown links, especially ones associated with social networks.

"As long as spammers continue to make money from these schemes, Internet users can be sure that they'll continue to receive unsolicited emails and social networking scams," Graham Cluley, senior technology consultant at Sophos, said in a statement. "To combat this, it's essential that computer users remain wary of clicking on unknown links, regardless of whether they appear to be on a trusted contact's social networking page."