The U.S. accounted for 23.2 percent of all spam spewed out in the second quarter, said Sophos, with China close behind at 20 percent. The third through fifth spammers -- South Korea, France, and Spain -- generated only 17.5 percent.
Monday's figures went counter to a steady decline in U.S.-produced spam over recent years.
"Since the introduction of the CAN-SPAM [Act] in 2004, we've seen a regular quarter-on-quarter drop in the proportion of spam coming from the U.S.," said Graham Cluley, Sophos senior technology consultant, in a statement. "Until now, that is. Perhaps the reality is that the statistics can't be reduced any further unless U.S. home users take action to secure their computers and put a halt to the zombie PC problem."
Sophos, like most anti-virus and anti-spam software developers, lays the blame on zombies, compromised computers that attackers have hijacked, then turned over or leased to spammers, who use them to send junk mail so they can keep law enforcement at bay.
The company also added its voice to the growing chorus that image-based spam -- junk mail composed of embedded images rather than plain text -- is rapidly increasing in popularity. Sophos said that image spam accounted for 35.9 percent of all spam in June, nearly double January's 18.2 percent.
Spammers have turned to images in an attempt to slip by traditional anti-spam filters, which parse the text message to hunt for typically "spammy" words or phrases.
Pump-and-dump spam scams were also highlighted by Sophos, which said that the stock schemes now make up 15 percent of all spam, up from January 2005's less-than-1 percent and also up from the 13 percent in December. The scams, which hype a stock to push up the price, are effective, a pair of German researchers said earlier this year., and the increase spotted by Sophos backs up their findings.
"It's worrying to see so many pump-and-dump e-mails," Cluley said. "The people that act upon these e-mails aren't skilled investors, and don't realize that purchasing the shares is likely to reap no reward, benefiting only the spammers."