Botnets are now responsible for sending almost nine out of ten spam messages according to a recent report from security software provider Symantec.

Botnet is a term used by security specialists to describe a group of computers, called robots or bots, which run autonomously and automatically.

The term is most often associated with malicious software but can also refer to the a of computers using distributed computing software.  Evil botnets, the ones that send out spam, are often named after their malicious software name.

In Canada and the U.S. in September, spam accounted for 91.2 and and 91.8 percent of all email.

Symantec's most recent analysis has found that a newer botnet, called Maazben, has experienced rapid growth since began operating in late May - mainly sending out casino-related spam - while Rustock, one of the oldest and largest botnets, has doubled in size since June. 

Maazben’s growth accelerated in Septembers and was responsible for 1.4% of spam in September up from 0.5% in August. The company says Rustock is the largest botnet in terms of number of bots at an incredible 1.3 to 1.9 million bots but has kept its output per bot relatively low. 

Interestingly, the researchers say Rustock has settled into a predictable spam pattern beginning everyday at 3 a.m. ET, peaking at 7 a.m. ET and ceasing spamming at 7 p.m. ET. It then rests for eight hours before beginning again. Rustock is the only botnet with a regular spam cycle. One of the most dominant botnets, Rustock is responsible for ten percent of all spam. As such, its spam pattern is reflected in overall total daily spam patterns. 

Other large and active botnets are Grum, half the size of Rustock but responsible for 23.2 percent of spam, and Bobax, responsible for 15.7 percent of spam. 

Readers can find the the latest  MessageLabs Intelligence Report from Symantec online at