Just weeks after McColo's shutdown sent global spam volumes plummeting, researchers are in disagreement as to extent of the long-term effect on junkmail levels of the hosting-firm's demise (see also: Spam epidemic eliminated as ISP taken offline).

Some say that junkmail rates have remained significantly lower since McColo was disconnected from the web, while others say spam has already bounced back. All agree, however, that the infamous Srizbi botnet is gone for good, which will reduce spam levels.

The shutdown of California-based McColo, a company that hosted a staggering variety of cybercriminal activity, on November 11 cut spam by as much as 75 percent in the first few days after its upstream internet providers pulled the plug.

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The shutdown slashed spam volumes because some of the planet's biggest spam-sending botnets were controlled from servers hosted by McColo, according to security researchers who had long urged the company's disconnection from the web.

While spam initially slid off a digital cliff, two weeks later it's unclear whether spammers have resumed their usual practices.

A researcher with IronPort Systems, a messaging security company owned by Cisco Systems, today said that spam is still down, if not out. According to IronPort, Tuesday's spam volume was approximately 72.7 billion messages, less than half of the 153 billion on November 11, but up from the 64.1 billion of November 13, two days after McColo went off the air.

"We're seeing small spikes in spam volumes relative to the post-McColo shutdown volumes," said Nick Edwards, a senior product manager at IronPort, in an email Tuesday explaining the uptick.

"We believe the spammers are trying other botnets - those whose command-and-control infrastructure and front-end applications were not hosted by McColo."

They're not having much luck, Edwards added.

"Spam volumes are still down significantly," he said. "While there was a temporary increase in spam volume [last] Friday and Saturday, spam volumes have not approached levels prior to the McColo shut down. The spammers are having a difficult time finding a botnet for lease that they can use effectively."

Researchers at rival MessageLabs Group - now part of Symantec - see the situation differently.

According to Matt Sergeant, a senior anti-spam technologist at the company, spam levels have bounced back to about two-thirds of what they were before McColo was yanked off the internet. In fact, spam jumped to that volume only today.

Sergeant wasn't surprised by the lag time between McColo's shutdown and a return of spam.

"The Asprox and Rustock botnets are back with a vengeance after having found new command and control [servers]," Sergeant said in an email.

"Cutwail never went away and it seems its owners have used the opportunity to increase output. Mega-D is also on the rise again."

Sergeant and Edwards, however, agreed on one thing: the Srizbi botnet looks gone for good.

"Srizbi, having once been responsible for 50% of all spam, is now completely defunct," said Sergeant, who added that sans that botnet, "spam levels won't return to what they had been."

Edwards confirmed that Srizbi was still offline.

"And we have confirmation that McColo traffic has not been re-hosted somewhere else," he added.

"The backers of both are still scrambling." McColo was still unavailable as of mid-afternoon Tuesday.

Srizbi, which also goes by "Mailer Reactor," was among the world's biggest botnets. In April, noted botnet researcher Joe Stewart of SecureWorks Inc. estimated Srizbi as composed of 315,000 infected PCs.

The McColo takedown, Stewart said last week, had cut off more than half a million compromised computers - aka "bots" - from their criminal controllers.

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