Botnet was responsible for 18 billion spam messages a day -- about 18 percent of the world's spam -- experts tell The New York Times.
The software giant reached an agreement with the owner of 3322.org, a site that has been linked to malware such as the Nitol botnet.
Discovery leads to investigation and disruption of Nitol botnet and attempt to shut down subdomains linked to more than 500 types of malware.
Spam levels increase more than 5 percent in February over prior month, due in large part to greater activity from Grum and Rustock botnets, according to Symantec.
Spam made up more than 90 percent of all e-mail last month, with networks of zombie PCs producing the vast majority of such messages, MessageLabs says.