In March, the Rustock botnet had been sending as many as 13.82 billion spam emails daily, accounting for an average of 28.5% of global spam sent from all botnets in March, and when the botnet ceased sending spam this month, global spam volumes fell by one-third, according to a new report by Symantec.

However, since the notable Rustock takedown, other botnets have stepped up their activities to take advantage of the gap in the market with Bagle now taken over from Rustock as the most active spam-sending botnet in 2011.

The MessageLabs Intelligence report identified that global spam volumes fell by 33.6% between March 15 and 17 following legal action against command and control hosts used by the Rustock botnet.

In the days following the takedown of Rustock, spam accounted for approximately 33 billion emails per day, compared with an average of 52 billion per day in the previous week.

By the end of 2010, Rustock had been responsible for as much as 47.5% of all spam, sending approximately 44.1 billion e-mails per day.

Since the end of 2010, the more-active Bagle botnet has sent approximately 8.31 billion spam emails each day, the majority linking back to pharmaceutical products.

In March 83.1% of global spam was sent from botnets, an increase of 6.1 percentage points compared with the 77% at the end of 2010.

The report revealed that in March 2011, the global ratio of spam in email traffic from new and previously unknown bad sources decreased by 2%; and the global ratio of email-borne viruses in email traffic from new and previously unknown bad sources was one in 208.9 emails (0.479 percent).

Further, phishing activity was 1 in 252.5 emails (0.396%), a decrease of 0.065 percentage points since February; and analysis of Web security activity showed that an average of 2,973 websites each day were harbouring malware and other potentially unwanted programs including spyware and adware, a decrease of 27.5% since February.

Geographically, Oman became the most spammed in March with a spam rate of 87.9 percent, in US 79.6% of email was spam and 79.4% in Canada, and spam level in the UK was 79.1%.

In The Netherlands, spam accounted for 80.2% of email traffic, while spam levels reached 80% in Germany, 78.9% in Denmark and 78.8% in Australia.