Spammers are out in full force getting ready for the festive season according to security company Postini.

The company has seen spam grow by nearly three-fifths over the last three months and said spam, which is increasingly criminal in intent, now accounts for nine in 10 of all emails sent.

Postini's executive vice president of marketing, Daniel Druker, said: "This dramatic rise in spam attacks… has the internet under a state of siege.

"Spammers are increasingly aggressive and sophisticated in their techniques, and protection from spam has become a front-burner issue again. Spam has evolved from a tool for nuisance hackers and annoying marketers to one for criminal enterprises."

The preferred method of delivery is still through the massive networks of hijacked computers called "botnets". Postini said each day it tracks more than one million infected computers that are co-ordinating spam and virus attacks, with 50,000 or more active at any instant.

The latest report from the anti-spam organisation Spamhaus, currently fighting a messy legal battle in the US brought by a company it accused of spamming, has updated its ROKSO list of known spammers. It said eight in 10 of the world’s junk mail comes from just 200 spam gangs.

These gangs are spread out around the world but Spamhaus said of the 10 worst individual offenders, four can be found in Russia, two in the United States and one in Israel, Canada, Hong Kong and the Ukraine.

These spammers are also continuously evolving their tactics to bypass spam filters. Image spam and MS Office document spam now makes up as much as three in 10 (30 per cent) of all junk messages, up from two per cent in 2005.

Image spam is where the hackers use techniques such as re-arranging as many as 25 tiny images into a message in an HTML email or using animated GIF attachments.

The aim is to bypass optical character recognition technology used by many email security systems. Infected computers are now also re-trying temporarily blocked email connections just like real mail servers do.