Spam continues to proliferate, despite the shutdown of major spam sources.

Google reports that the average volume of spam messages in the second quarter of 2009 was 53% higher than it was in first quarter of the year.

MX Logic, a Web and e-mail security company, reports a more significant increase during that period: It says spam volume increased 51% in June, 35% in May, and 40% in April. The company says that spam as an overall percentage of e-mail volume is at its highest point since December 2006 when image-based spam was at its peak.

There have been some recent declines in spam volume, prompted by the closure of ISPs hosting spam-related infrastructure. But those declines have swiftly been reversed as new spammers have stepped in to fill the void.

According to Amanda Kleha, a member of Google's message security team, the closure of the ISP McColo in November led to a 70% drop in spam volume. But within four months, as she explained in a blog post on Wednesday, spam volume had recovered.

Another ISP that served spammers, 3FN or Pricewert, was shut down by the FTC on June 4. Kleha says that reduced spam volume by 30%. "[I]t also created an open invitation for opportunistic spammers to once again seize a market opportunity," she said.

Since then, spammers have pushed spam volume up 14% from levels seen following the 3FN closure.

Kleha notes that spammers have recently been revisiting retro spam techniques. These include newsletters sent out with malicious links and images and image spam, which was big in 2007 but has declined as security vendors adapted. She speculates that this resurgence of old-style spam may reflect newcomers entering the business. She also acknowledges the possibility that it may be a way of testing the scope of current spam defenses.

June also saw a resurgence of payload viruses, otherwise known as malicious e-mail attachments. Kleha sees this as an effort by spammers to expand their botnets.

MX Logic expects that the coming July 4 holiday in the U.S. will prompt holiday-themed attacks.

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