Most people are not happy with their anti-spam products, according to a new survey.

From McAfee and Symantec to Apple and Microsoft, most anti-spam vendors are failing to fully satisfy customers, according to the survey by Brockmann & Company.

The best-performing technology by a large margin is made by challenge-response vendors like Sendio and SpamArrest, which challenge the identity of first-time senders, the report states.

But customers rarely are fully satisfied by anti-spam filters packaged with email clients, hosted email or commercial anti-virus software. Too often, the products let spam through and mistakenly delete email that's not spam.

Thirty six percent of companies surveyed have lost business because of legitimate emails getting caught in spam filters, said report author Peter Brockmann, president and research director.

"Whatever products they have developed obviously haven't been working," Brockmann said.

That's bad news, as PDF spam seems poised to overtake image spam as the next big problem. "Now it looks like there's going to be PDF spam, which is even worse for businesspeople," Brockmann said. "We sign purchase orders and pass contracts back and forth [as PDFs] all the time."

Rather than rate each vendor individually, Brockmann's survey divided technologies into eight categories. The firm surveyed 520 people working in IT, sales, marketing, finance, human resources and administration, or C-level executives.

The rate of customers who are not "very satisfied" is more than 70 percent for six of the eight types of anti-spam technologies. Commercial software filters, such as those produced by McAfee, Symantec and Trend Micro, fully satisfy just 22 percent of users, the report found. Filters that come with email clients, like those from Apple, IBM Lotus and Microsoft, fully satisfy only 21 percent of customers.

Satisfaction rates are similarly low for business-class email hosting providers, filtering appliances, and reputation systems known as "real-time black lists" from Commtouch, IronPort and Spamhaus.

The worst-performing technology appears to come from open source projects like SpamPal and SpamAssassin, which fully satisfy just 16 percent of users.

The most-satisfied customers use challenge-response vendors, which fully satisfied users 67 percent of the time.

Challenge-response tools allow messages from known senders without interruption, since virtually all spam comes from first-time senders. First-time senders are challenged with a reply email telling them to reply, click on a URL, or visit a website to complete delivery of the message.

"This procedure overcomes the weakness of spammers since spammers never monitor the reply-to accounts of their messages," the Brockmann report said.

Hosted email filtering services such as Google-Postini, AppRiver, and MXLogic performed second best, as customers report being very satisfied 42 percent of the time. These services use the processing power of Internet data centres to scour enormous quantities of email and find suspicious messages, Brockmann said.

People get an average of 11 spam messages per day, accounting for 15 percent of all messages, Brockmann said. That's after the work done by spam filters. Before filtering, probably 90 percent of email is spam, he says.

Brockmann has also devised a spam index that lets users calculate a score based on the amount of spam emails they get, the number of trapped messages, minutes per day dealing with spam, and estimated number of resend requests. Challenge-response vendors performed the best by a large margin under this measure as well, while business-class email hosting service providers did the worst.