Security firm FireEye has claimed it dismantled a botnet responsible for up to a third of the world's spam.
All known command-and-control servers for the Grum botnet have been removed in Panama, Russia and the Netherlands after community pressure on the internet service providers in those countries hosting them, according to the company.
Anti-spam group Spamhaus reported that Grum's botnet comprised around 120,000 'zombies', or compromised machines sending spam, every day.
The figure had since been reduced to 21,505 machines, according to FireEye, and may go away altogether as the spam templates on the now-orphaned zombies expire.
The Grum system also hosted websites on infected machines, some of which were found to be on corporate networks.
At its height in January this year, the botnet was estimated to be responsible for more than a third of the world's spam — and the most prolific network globally.
Before its closure, it was believed to be sending approximately 17 percent of global spam traffic, making it the third most active botnet in the world.
Security researcher Brian Krebs traced the Grum botmasters in February this year, which he believed to be a Russian hacker nick-named GeRa or Ger@.
Krebs estimated the SpamIt spam program, which Grum is believed to belong to, generated $US150 million in revenue over four years.
He said that businesses had to spend an estimated $US40 million annually to combat the junk mail.