Six botnets are responsible for 85 per cent of all spam, according to an analysis by net security firm Marshal.

The Srizbi botnet is reckoned to be the largest single source of spam - accounting for 39 per cent of junk mail messages – followed by the Rustock botnet, responsible for 21 per cent of the spam clogging up users' inboxes.

Spam emanating from the Mega-D botnet, which Marshal reckons was the leading source of junk mail in early February, was temporarily stemmed after control servers were taken out in mid-February. The estimated 35,000 zombie clients associated with the Mega-D botnet were infected with the Ozdok Trojan.

After 10 days of inactivity, spam from compromised hosts began flowing again earlier this week, after hackers re-established control. Despite the break in transmission, Spam-D accounted for an estimated 11 per cent of junk mail hitting Marshal's spam traps during February.

Other active spam botnets include Hacktool.Spammer (AKA Spam-Mailer) and botnets associated with the Pushdo (AKA Pandex) family of malware.

The notorious Storm botnet, estimated to include about 85,000 compromised hosts, is thought to be responsible for only three per cent of spam.

"The size of a botnet, measured by how many bots it has, does not necessarily correlate with how much spam it sends. Our team has observed huge variations in the rate at which different spambots pump out spam," said Bradley Anstis, VP of products at Marshal.

In many instance, spammers have access to multiple botnets. In addition to Mega-D, other botnets - including Srizbi, Rustock, Hacktool.Spammer and Pushdo - have been simultaneously sending spam promoting Express Herbals, a line of male enhancement pills.

According to February statistics from managed security firm Network Box, the US continued to pump out the most spam and spread the most viruses. The country accounted for 13 per cent of all viruses; and was the source of 15 per cent of all spam, more than double its closest junk mail rival, Turkey. ®