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Online industry analysts agree that while opt-in e-mail is a relatively small corner of the electronic commerce market, its low operating costs, compared with print direct mail, can make it a valuable marketing tool.

"If you can target-messages effectively and have a strategy to use them, obviously the return on those [e-mail] programs are astronomical because your base is zero," said Scott Nelson, an analyst at Gartner Group, Inc. in Stamford. Conn.

Bill Bass, an analyst at Forrester Research, agreed, pointing to several airlines' use of opt-in E-mail as a natural fit. "Airlines have real problems with inventory control," he said. "The expiration date on their product is absolute and short-term. On products like that, [e-mail] is great. And it's also a great deal for the consumer, its win-win both ways."

For customer retention, Bass added, opt-in E-mail is "one of the greatest things since sliced bread. You can communicate with your customer base extraordinarily cheaply, and it's one-to-one marketing that the customer asked for, so its nonintrusive."

REI, a Seattle-based outdoor equipment firm with about $500 million in sales last year, sends its Gearmail subscribers regular e-mails on products, upcoming sales and other events. REI online store manager Mart Hyde said sales from REI's Web site have topped sales from some of its smaller retail outlets.

Gearmail runs off a Microsoft Access database and anIBM RS/6000 server. REI plans to boost productivity by integrating the database with an enterprisewide, proprietary customer data system that will be networked with REI's stores, mail-order service and Web site, Hyde said. Nelson confirmed that opt-in e-mail is most productive when it is fully connected to a company's back-end system. "The optimal [scenario] would be to have it tied to your inventory and manufacturing system such that you had triggers that said, 'When these events occur, we want to notify our customers.' "

The big question is who decides what should trigger a mailing: the business or the consumer. "With event-driven computing, either the person sending the information can set up the rules about the event or the end user can," said Maureen Fleming, a Gartner analyst. "In my mind it's always, 'Let the end user win' because then you have a more qualified lead."

All of which is easier said than done. Case in point: Travelocity, a Web site owned and operated by The Sabre Group, notifies FareWatcher subscribers whenever a fare they're interested in drops more than $25. But subscribers also receive targeted promotions they didn't ask for. Sabre Chief Information Officer Terry Jones maintained that his mailings are "relatively low intrusion" because FareWatcher subscribers volunteer their e-mail addresses when they sign up. "We can send literally hundreds of thousands of e-mails and get maybe 10 people who say, 'Please don't do that again,' " he said.

Forrester's Bass doesn't buy Jones's justification. "If I opt in and all of a sudden you start sending me a whole bunch or advertising things, that's spam," he said. Gartner's Nelson agreed that low negative response is no guarantee a mailing has been successful. "Sending customers information they're not really interested in only makes them less sensitive to future offers," he said.

But Nelson is more sanguine than Bass about how far businesses can push the "opt- in" envelope, suggesting it can be stretched to include the use of information customers provide over time. "There's lots of times you can use the information, but you have to soft-pedal that you know as much as you do about your customers or they start to become very nervous," he said.