Small businesses are more likely to be targeted by spam email than larger companies. According to Postini, an email security firm, businesses with 100 or less computer users get up to 10 times more spam than corporates employing over 10,000 workers.

It says that small businesses lacke the budget to invest in the latest anti-spam software. As spammers adoopt increasingly sophisticated ways of getting through company filters, small firms are more likely to be affected.

Chris Smith at Postini said: "What we're seeing is a profound increase in the sophistication and incidence of tactics designed to fool conventional anti-spam filters. Spammers are also finding smaller companies more susceptible to attack since they typically have fewer and less sophisticated defences in place than larger enterprises."

Postini's study shows that around one per cent of all spam is some form of phishing - emails designed to trick users into revealing passwords and other sensitive information. One in three unsolicited messages are sent by zombie networks which use innocent victims' computers as a conduit for delivering spam to other users.

Businesses in the publishing, advertising and legal sectors proved more prone to attack from unsolicited messages, while banking, financial, manufacturing and pharmaceutical organisations suffered least.

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