Spammers came out in force last month generating a deluge of unsolicited email that "severely threatened corporate networks", a monitoring report claimed today.

According to Postini's analysis of almost 70 billion emails from September to November, there was a 59 per cent spike in spam over the period.

The firm reported that unwanted email currently represents 91 per cent of all email, and that the daily volume of spam has risen by 120 per cent over the past 12 months.

Postini also reported a "dramatic increase" in overall email traffic, with 10 billion more communications in October than in September.

"This dramatic rise in spam attacks on corporate networks has the internet under a state of siege," said Daniel Druker, executive vice president of marketing at Postini.

"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."

He added that spammers are increasingly using hijacked botnets to initiate ever more aggressive attacks.

Postini is currently tracking more than one million infected computers that are coordinating spam and virus attacks each day, with 50,000 or more active at any time.

Spammers are also continuously evolving their tactics. Image spam and Microsoft Office document spam now make up as much as 30 per cent of all junk messages, up from two per cent in 2005.

Hackers are also using techniques such as re-arranging as many as 25 tiny images into a message in an HTML email, or using animated GIF attachments to dodge optical character recognition technology.