Some popular free Web services are playing both sides of the fence when it comes to protecting consumers from pesky marketers, offering to block junk email while they help advertisers push promotions into customers' in-boxes.

The practice underscores a delicate balancing act for email services, some of which tout features for filtering unwanted messages while they 'cozy up' to advertisers with "opt in" email programs - for a fee.

Those deals fall short of supporting the most reviled form of online marketing messages: unrequested bulk email known as spam. But anti-spam advocates say Web services that offer opt-in mailing lists are walking into a marketing grey area - one that could grow increasingly murky as the practice becomes more popular.

In this battle, analysts predict that spam filters, ostensibly aimed at protecting consumers from junk email, will increasingly play a role in turning promotions into profits for Web email providers.

"The hypocrisy is that these companies want to stop junk email from other parties but at the same time want to exploit their own customers to the greatest extent possible," said anti-spam advocate Jason Catlett, of Junkbusters.

"Consumers need to realise that there is no such thing as a free lunch."

Opt-in programs have been in place for some time, and analysts predict they will become more pervasive in the coming months as free Web services search for new revenue streams.

In a study released last week, New York-based Jupiter Research, a division of Jupiter Media Metrix, predicted that Web email providers and Internet service providers will increasingly use their power to weed out unwanted messages from consumer email in-boxes as a way to squeeze profits out of direct email marketers.

Since Web email services already restrict access to a member's primary in-box, advertisers looking to reach consumers online will be forced to pay a premium to avoid getting dumped in the trash, according to Jupiter analyst Christopher Todd.

"Consumers are going to complain about the increased spam, so that's why we'll see a shift from the primary in-box to the bulk email box," he said.

Online portal Yahoo!, for example, gives email subscribers the opportunity to filter bulk email, directing certain messages to a junk email folder rather than to the main in-box. Jupiter projects that such services will get modified so members can select topics they wish to hear about, allowing related bulk messages into their primary in-boxes.

The hitch is that marketers will have to pay to be on the select list. Advertisers could shell out an even higher premium to reach consumers based on the time they read their email, according to Jupiter.

Yahoo! said it is has been taking advantage of its relationship with marketers and email subscribers for two years.

"We've recognized the value of online direct marketing for quite a while," said Nicki Dugan, a Yahoo! spokeswoman.

Since some people may request marketing notices that appeal to their interests, the company sees no problem with the practice.

The online portal asks consumers when they sign up for an email account if they would like to receive promotions based on their preferences. If the consumer opts in, it will charge marketers to reach these consumers via email.

Microsoft's Hotmail also offers a primitive form of this type of paid marketing through its Web courier email service. Newsletter publishers pay to be on a list that consumers can choose from to receive content via email.

The publisher then sells advertising on the email newsletters to recoup losses on delivery.

Junkbusters' Catlett said opt-in programs that gain genuine permission from consumers to send material are kosher and raise no concerns of double-dealing. But he cautioned that not all opt-in programs are alike, with some aimed at duping

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