Marshal Security says that its research team have identified that a total of six botnets are responsible for 85 per cent of the spam in circulation on t'Internet.
Three weeks ago, Marshal says it reported the Mega-D botnet was the leading source of spam.
Following the announcement, researchers identified the malware behind the 35,000-strong botnet as Ozdok. The subsequent discovery of Mega-D's control servers saw spam sent from this botnet drop to zero during mid-February, says the company.
This week, however, Marshal says that Mega-D returned again to represent 21 per cent of spam after a 10-day period of inactivity.
Owing to the break, Mega-D only accounted for an average of 11 per cent of spam during February.
At its peak last month, however, the firm says it was responsible for a third of all the spam we caught in our spam traps.
"While the recent publicity spooked the Mega-D spammers into taking their control servers offline, they have now clearly re-established themselves elsewhere," said Bradley Anstis, Marshal's vice president.
According to Anstis, the size of a botnet, measured by how many bots it has, does not necessarily correlate with how much spam it sends.
"Our research team has observed huge variations in the rate at which different spambots pump out spam," he said, adding that some spammers may have access to multiple botnets.
Check out more on botnets and spam here...