Two years ago, right before Carly Fiorina took the helm at Hewlett Packard, its Web site was rated one of the worst on the Internet: HP was known for its focus on equipment, not customers. Last January, to improve its relationship with clients, HP sent e-mail newsletters to all customers who registered online. It gave new customers incentives like better warranties to register via its Web site. Implementation of the campaign was left to Digital Impact Inc., a Silicon Valley e-marketing company.

Digital Impact segmented HP’s customer e-mail list by date-of-purchase and product, and sent customized newsletters containing information about optimizing the performance of HP products and services, including reminders about maintenance.

Here’s the cool part: The newsletters featured a button allowing readers to forward it to friends or colleagues. When a user makes a click and sees a webpage that is hosted by Digital Impact for HP, that’s where they can put in the friend’s e-mail and a note, and then push the send button,” says Wiebke Sinzliu, b2b engagement manager for Digital Impact. The technology takes the message, inserts it above the newsletter, and e-mails the whole thing to the friend.”

At the bottom of each newsletter readers are asked if they’d like to receive HP newsletters themselves. If so, they can sign up , adding their name to HP’s database.

Though HP’s marketers may not have realized it, having its customers send newsletters to friends or colleagues is textbook viral marketing, one of the newest, cheapest ways of marketing on the Web. It involves creating an e-mail so compelling either graphically or by using an incentive that customers want to pass it along. And when e-mail comes from a friend, the recipient is much more likely to open and read it.

Hewlett Packard’s goal with this campaign was to drive consumers to its Web site and ultimately to increase sales. Viral success is measured in click-troughs how many readers clicked on the teaser of a newsletter article, hooked into HP’s Web site, and read the full article.

For the people from our original list, the click through rate was 10% to 15%,” mentions Sinzliu. The ones that received it from a colleague or a friend, the click through rate was between 25% and 40%.” HP’s numbers closely reflect the national data for viral marketing. Industry analysts claim between five and fifteen percent of the people that receive viral messages actually click the links. The argumentation for the better results among those obtaining messages virally is that the audience is already pre-qualified.

This is true especially in the b-to-b arena,” Sinzliu says. Often someone sends it to a colleague after they’ve talked about something similar, perhaps in connection with a work-related problem.”