It's easy to see how phishing, the illegal practice of using e-mail to extract sensitive information from unsuspecting users, is becoming mainstream: Count the number of messages in your in-box that purport to be from eBay.

You also could tally the number of services on the market designed to protect in-boxes from these malicious e-mails.

According to the Anti-Phishing Working Group, in March there were 18,480 incidents of phishing reported. In April that number dipped slightly to 17,490, then rebounded to an all-time high of 20,109 in May. Much like the anti-spam market that exploded earlier this decade, the number of security companies with services designed to protect people from phishing attacks has grown rapidly over the past few years.

Compounding the problem is the fact that phishing can do serious financial damage to consumers and corporations. While spam carries its own costs in terms of bandwidth, e-mail storage and lost productivity, phishing attacks can extract data that is sometimes so sensitive it's impossible to put a price tag on it (intellectual property, for example).

Making matters worse is the fact that phishers have enough financial incentive to develop complex, obscure networks of zombie PCs to do their phishing that they are staying one step ahead of vendors.

"No one vendor can catch all the phishing attacks," not even AOL, which does the best job of protecting its subscribers, says Avivah Litan, vice president and research director at Gartner.

Nonetheless, the security market is responding. Anti-phishing products designed for consumer use have been available for a few years, such as tools from AOL and EarthLink that help protect their subscribers from the threat. Protection for enterprises has taken longer to evolve - perhaps because phishing has been viewed as a consumer problem - but today many security vendors offer phishing protection either by blocking malicious e-mail from entering users' in-boxes, or by halting access to URLs that are known phishing sites. While such features are touted by makers of Web- and e-mail-filtering software and appliances, some say the most effective way for enterprises to prevent phishing is through a hosted service that never lets the dangerous e-mail onto a corporate network to begin with.