That's why we get so much junk e-mail: It's essentially free to send. So Microsoft Corp. chairman Bill Gates, among others, is now suggesting that we start buying "stamps" for e-mail.
Many Internet analysts worry, though, that turning e-mail into an economic commodity would undermine its value in democratizing communication.
But let's start with the math: At perhaps a penny or less per item, e-mail postage wouldn't significantly dent the pocketbooks of people who send only a few messages a day. Not so for spammers who mail millions at a time.
Though postage proposals have been in limited discussion for years - a team at Microsoft Research has been at it since 2001 - Gates gave the idea a lift in January at the World Economic Forum in Davos, Switzerland.
Details came last week as part of Microsoft's anti-spam strategy. Instead of paying a penny, the sender would "buy" postage by devoting maybe 10 seconds of computing time to solving a math puzzle. The exercise would merely serve as proof of the sender's good faith.
Time is money, and spammers would presumably have to buy many more machines to solve enough puzzles.
The open-source software Hashcash, available since about 1997, takes a similar approach and has been incorporated into other spam-fighting tools including Camram and Spam Assassin.
Meanwhile, Goodmail Systems Inc. has been in touch with Yahoo! Inc. and other e-mail providers about using cash. Goodmail envisions charging bulk mailers a penny a message to bypass spam filters and avoid being incorrectly tossed as junk.
That all sounds good for curbing spam, but what if it kills the e-mail you want as well?
Consider how simple and inexpensive it is today to e-mail a friend, relative or even a city-hall bureaucrat. It's nice not to have to calculate whether greeting grandma is worth a cent.
And what of the communities now tied together through e-mail - hundreds of cancer survivors sharing tips on coping; dozens of parents coordinating soccer schedules? Those pennies add up.
"It detracts from your ability to speak and to state your opinions to large groups of people," said David Farber, a veteran technologist who runs a mailing list with more than 20,000 subscribers. "It changes the whole complexion of the Net."
Goodmail chief executive Richard Gingras said individuals might get to send a limited number for free, while mailing lists and nonprofit organizations might get price breaks.
But at what threshold would e-mail cease to be free? At what point might a mailing list be big or commercial enough to pay full rates? Goodmail has no price list yet, so Gingras couldn't say.
Vint Cerf, one of the Internet's founding fathers, said spammers are bound to exploit any free allotments.
"The spammers will probably just keep changing their mailbox names," Cerf said. "I continue to be impressed by the agility of spammers."
And who gets the payments? How do you build - and pay for - a system to track all this? How do you keep such a system from becoming a target for hacking and scams?
The proposals are also largely U.S.-centric, and even with seamless currency conversion, paying even a token amount would be burdensome for the developing world, said John Patrick, former vice president of Internet technology at IBM Corp.
"We have to think of not only, let's say, the relatively well-off half billion people using e-mail today, but the 5 or 6 billion who aren't using it yet but who soon will be," Patrick said.
Some proposals even allow recipients to set their own rates. A college student might accept e-mail with a one-cent stamp; a busy chief executive might demand a dollar.
"In the regular marketplace, when you have something so fast and efficient that everyone wants it, the price goes up," said Sonia Arrison of the Pacific Research Institute, a think tank that favors market-based approaches.
To think the Internet can shatter class distinctions that exist offline is "living in Fantasyland," Arrison said.
Nonetheless, it'll be tough to persuade people to pay - in cash or computing time that delays mail - for something they are used to getting for free.
Critics of postage see more promise in other approaches, including technology to better verify e-mail senders and lawsuits to drive the big spammers out of business.
"Back in the early '90s, there were e-mail systems that charged you 10 cents a message," said John Levine, an anti-spam advocate. "And they are all dead."
By Anick Jesdanun