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Postini said the threat to smaller firms, which usually have less network protection, comes at a time when spam is making up between 75% and 80% of e-mails being sent to all employees.
Postini classes a small firm as one with less than 100 employees. Postini said some industries, including publishing, advertising, legal firms, and estate agents, were particularly susceptible to spam, because they widely publicise the e-mail addresses of their employees.
These market segments received 10 times more spam than those companies in banking, manufacturing, electronics, food and beverages, and pharmaceuticals.
Virus-infected e-mails tripled as a percentage of all e-mails, making up 1.5% of e-mails in 2004, up from 0.5% in 2003. As much as 1% of all spam is now part of some phishing attempt, Postini said.
Matt Cain, an analyst at Meta Group, said, "More sophisticated, phishing-style attacks will proliferate as bulk spamming scams decrease in effectiveness.
"We also see dynamic, fast moving threats such as zombie networks posing particular challenges to corporate systems." Zombie attacks use the computers of unaware third parties to unleash spam and viruses.