A botnet that was once responsible for an estimated third of the world's spam has been knocked out of commission thanks to researchers from security firm FireEye.

After carefully analyzing the machinations of the massive botnet, alternately known as Mega-D and Ozdok, the FireEye employees last week launched a coordinated blitz on dozens of its command and control channels. The channels were used to send new spamming instructions to the legions of zombie machines that make up the network.

Almost immediately, the spam stopped, according to M86 Security blog. Last year, the email security firm estimated the botnet was the leading source of spam until some of its servers were disabled.

The body blow is good news to ISPs that are forced to choke on the torrent of spam sent out by the pesky botnet. But because many email servers already deployed blacklists that filtered emails sent from IP addresses known to be used by Ozdok, end users may not notice much of a change, said Jamie Tomasello, an abuse operations manager at antispam firm Cloudmark.

The takedown effort is significant because it shows that a relatively small company can defeat a for-profit network that took extraordinary measures to ensure it remained operational. Not only did Ozdok reserve a long list of domain names as command and control channels, it also used hard-coded DNS servers. When all else failed, its software was able to dynamically generate new domain names on the fly.

With head chopped off of Ozdok, more than 264,000 IP addresses were found reporting to sinkholes under FireEye's control, an indication of the massive number of zombies believed to have belonged to the botnet. FireEye researchers plan to work with the ISPs to identify the owners of the orphaned bots so their owners can clean up the mess.

FireEye researchers said the key to dismantling the giant ring was a coordinated effort that worked in multiple directions all at once so that bot herders didn't have a chance to counteract. "As it turns out, no matter how many fallback mechanisms are in place, if they aren't all implemented properly, the botnet is vulnerable," they wrote. ®