Spam Cooking Over at Yahoo!, AOL

"Volume senders will use this because it makes good business sense," says Gingras in an interview. "They will get a very significant return on their investments. Their delivery will be dramatically improved."

But privacy expert Ray Everett-Church says he believes the new service unfairly penalizes legitimate companies.

"Unless the major ISPs are going to require postage for 'every' email coming into their system, they will still need to accept, sort and deliver unpaid email from the rest of the world," Everett-Church says. "At that point, they'll still be dealing with massive volumes of unwanted email. The only difference is that they'll be able to offset those costs by taxing legitimate sources of email. Doesn't sound fair to me, but the world of spam is filled with inequities and bad decisions."

Priceline (PCLN Quote) and Expedia (EXPE Quote) declined to comment. Other companies, including  United Online (UNTD Quote), are taking a wait-and-see approach.

"While it is too early to know exactly how these efforts will affect United Online, the goal of any actions we take will be to ensure that our members continue to receive necessary communications from Classmates, PhotoSite and our other brands in a timely fashion," the Woodland Hills, Calif.-based company said in an email statement.

Though consumers aren't subject to the email fees, they may be in the future.

"These e-mail providers have been spending an enormous amount of money on building and maintaining their email transport structure," says Sara Radacati, the head of a research firm that monitors the email market. "Everybody is going to be charged eventually."

By 2009, there will be 2.2 billion email boxes, up from 1.4 billion this year, according to Radicati. During that same period, worldwide spam traffic will almost double by 2009 to 228 billion messages. Overall email traffic will jump 93% from 171 billion messages per day in 2006, to 331 billion messages per day in 2009.