A new plan from America Online and Yahoo to accept money from marketers in exchange for a promise not to block their e-mail has kicked up a firestorm of criticism, but it's unclear how this controversial system will affect spam levels for users of those services.

The ISPs say they are going to start requiring marketers to buy electronic versions of postage stamps to ensure that their e-mail makes it into AOL- and Yahoo-managed inboxes.

The libertarian-minded Internet community has given the plan a resounding thumbs down, arguing that charging people to send e-mail is like charging them for the air they breathe. Marketers say that charging for access to inboxes could hurt small businesses and Internet retailers who can't afford to pay the fees.

AOL and Yahoo say the new system allows them to protect their users from Internet threats and abuses. Because e-mail sent using this scheme would be visually distinguished as approved mail, certain order transactions, newsletters, and opt-in marketing messages would no longer be mistaken for spam and blocked by e-mail filters.

The technology behind the companies' bid to charge bulk e-mailers was developed by Goodmail Systems. It uses a list of companies that are allowed to send e-mail to Goodmail customers, similar to a whitelist. Companies that want to send bulk e-mail to AOL or Yahoo users purchase cryptographically secure stamps from the ISP and attach them to their outgoing messages. AOL and Yahoo use Goodmail's technology to detect, validate, and clear the stamps.

Stamped e-mail bypasses spam filters run by the ISP that issued the stamp. E-mail account holders could block the messages using their own desktop software, but would have to configure their filters themselves.

This doesn't mean that the ISPs will block all commercial e-mail that is not Goodmail certified. Marketers will still be able to send their e-mail to AOL and Yahoo customers, but their messages could be blocked by the ISPs' spam filters.

ISPs using the Goodmail system don't charge consumers for sending e-mail; fees are levied on bulk e-mailers only, who would be forced to pay US$2 to $3 per 1,000 messages. Analysts say fees paid to AOL and Yahoo could be a significant new revenue stream for each company.

Goodmail is responsible for making sure the companies that send e-mail using its technology are reputable. It does this by performing background checks on potential customers. The firms must have been in operation for at least one year; they must have low e-mail complaint levels with spam watchdog groups; and they must comply with Goodmail's acceptable use policy. The participating ISPs will have the final say on whether to permit a company's e-mail.

Getting e-mail past an ISP's spam filters is a big deal. Increasingly, ISPs are fighting spam by aggressively blocking massive amounts of suspicious e-mail. Unavoidably, legitimate e-mail gets blocked right along with blatant spam and phishing messages.

An AOL representative told me that the ISP blocks 80 percent of the spam it receives before the e-mail enters AOL's network. That includes junk e-mail that doesn't comply with federal anti-spam laws, virus-laden messages, and phishing scams. AOL wouldn't say how much, but admitted that legitimate marketing e-mail sometimes gets blocked accidentally. Personally, I can live with ISPs deleting unsolicited bulk e-mail hawking vitamins, travel specials, and anti-spyware software. Denver e-marketing firm Return Path claims that one in five "permissioned" messages (that is, those requested by the recipients) gets blocked before it reaches an inbox.

I'm concerned that once AOL and Yahoo implement the Goodmail technology and start collecting money from marketers, they will block less spam. After all, the more certified e-mail that AOL and Yahoo gives access to, the more the ISPs stand to profit. And for marketers, the Goodmail system is great because they can be assured their messages make it into consumers' inboxes. I predict this means I'll have to delete more e-mail that I consider spam from my AOL and Yahoo inboxes.

Another potential problem arises when companies don't want to pay an ISP's e-mail fees. For AOL and Yahoo customers this creates two classes of e-mail: a trusted class, and an unwashed class that hasn't been verified. This leaves AOL and Yahoo e-mail recipients in a bit of a quandary: If they can always trust Goodmail-certified messages, does that mean they should be suspicious of any other e-mail? That could mean that any organization - be it a charity, school, or bank - could be increasingly pressured to use the Goodmail system to send e-mail to AOL and Yahoo customers.

AOL and Yahoo say that using Goodmail's technology will help them identify legitimate messages and cut down on junk e-mail, identity-theft scams, and other e-mail related problems that plague users of their services.

The ISPs insist that when bulk e-mailers are forced to pay fees for the messages they send, they'll be less inclined to send out e-mail blasts to millions of people. Instead, marketers will be coaxed into better managing their lists, removing the addresses of those who have never responded to e-mail, and being more discriminating about who they send messages to in order to save money.

Goodmail points out that banks and other companies whose customers are frequent phishing targets will benefit. Customers will know when to trust e-mail from their bank or from a company like eBay, should it participate. Nonetheless, I don't want an ISP making it easier for advertisers to clutter up my inbox.

Ultimately, Goodmail's system benefits the ISPs that use it; it does little to combat spam. A new certified class of e-mail does nothing to reduce the cost of fighting spam, nor does it lighten the spam load.

AOL's and Yahoo's adoption of Goodmail technology underscores the fact that the industry is still trying to figure out how to address the spam problem. My only hope is that the potential millions of dollars in new revenues that the ISPs reap will be used in a more earnest fight to block spam.