Small businesses are experiencing much heavier spam attacks than larger companies, newly published research has claimed.

The study by email management and security firm Postini found that, although the cost of spam prevention reached a high point in 2004, threats to email systems grew worse with spam accounting for between 75 and 80 per cent of total email volumes.

Smaller companies, such as those with 100 users or less, received up to 10 times more spam per user than large businesses with 10,000 users or more, according to the study.

The spam plague was particularly severe for industries including publishing, advertising, legal and estate agency.

These firms received more than 10 times the amount of spam per user per day than organisations in the banking, financial, manufacturing, electronics, foods and pharmaceuticals sectors.

More than one third of all spam originated from zombie networks that use innocent victims' computers as a conduit for delivering spam.

Virus attacks trebled, with virus infected emails accounting for 1.5 per cent of all emails in 2004, up from 0.5 per cent in 2003. As much as one per cent of all spam attempted some variety of phishing attack.

"What we're seeing is a profound increase in the sophistication and incidence of tactics designed to fool conventional anti-spam filters," said Scott Petry, founder and senior vice president of products and engineering at Postini.

"Spammers are also finding smaller companies more susceptible to attack, since they typically have fewer and less sophisticated defences in place than larger enterprises."

Matt Cain, industry analyst with Meta Group, added: "More sophisticated phishing-style attacks will proliferate as bulk spamming scams decrease in effectiveness. We see dynamic, fast moving threats such as zombie networks posing particular challenges to corporate systems."