The boxbe service officially launched about a week ago, says Loder, and has accumulated more than 1,000 users. In particular, he says, boxbe could be useful to small-to-medium-size marketers, such as local businesses, that couldn't otherwise afford to launch an e-mail campaign. "A pizza-shop owner could come to our website, and, using a credit card, they can set up an account," Loder says. The owner could target boxbe users within a certain zip code.

At this point, however, no marketers are using boxbe for e-mail campaigns because the audience is too small to be useful, Loder says, although there has been some interest. He hopes to gather many more users during the next four months--enough to start reaching out to mainstream marketers. "The real sweet spot will be one million users," he says. "At that point the marketers will come to us."

Acquiring a critical mass of users, however, may be the biggest challenge that boxbe will need to overcome, says Hal Varian, professor of business, economics, and information management at the University of California, Berkeley. "If a pay-to-send service like boxbe were universally deployed, it would likely be a successful way to fight spam," he says. "But it's hard for a single company to get the critical mass, since existing companies that provide e-mail services are unlikely to acquiesce to a new firm getting between them and their users."

MacKie-Mason of the University of Michigan says that the service might be confusing to those who are less technologically savvy. However, at some point, the initial hassle of keeping track of a white list could be mitigated by being spared the masses of unwanted e-mail, MacKie-Mason says. "It may be that an e-mail world where we have to maintain a white list is better than a world flooded with spam."

According to Loder, e-mail advertising is an underused marketing tool. "People spend 50 percent of their time online using e-mail," he says. "But the amount of money spent to reach people over e-mail is a little over two percent of [the budget for] online advertisements. It's because legitimate advertisers don't want to be perceived as spammers." If boxbe catches on, he says, e-mail advertising could lose its stigma.