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Anti-spam technique: Postage
Date of first use: 1980s
Effectiveness: Unknown
Popularity: Low
Difficulty of implementation: Difficult to impossible
Where implemented: MTA or gateway
Harm: High

Spam is possible because the costs are asymmetrical. Sending mail costs much less than receiving it, so that receivers willingly subsidize legitimate senders, and unwillingly subsidize spammers. If it were possible to transfer some of the costs from receivers back to senders, that would presumably deter spammers, few of whom would be willing to pay the actual costs of their mail.

Some early proprietary closed mail systems such as MCI Mail and Compuserve charged per message, but charging disappeared when they became linked into the Internet's flat rate mail system. None ever handled inter-system settlements; for example MCI would charge its users for messages sent to the Internet, but would receive Internet messages for its users for free.

Postage on electronic mail is a highly contentious topic. Typical proposals involve providing stamps to senders which are collected by recipients, or by intermediate post offices on behalf of recipients. Often, recipients can decline payment for mail from known or preferred senders. All assume strong enough sender authentication to prevent spammers from avoiding paying postage by pretending to be someone else.

Some argue that it's flatly impractical to implement across the Internet, due to the costs and complexities of handling the enormous volume of mail, most of which is spam that won't pay anything, of operating the banks or post offices that would manage the payments, and of handling the range of errors and frauds that would arise. See the white paper referenced below for a longer analysis.

Goodmail Systems' Certified Email used a form of postage to provide preferred delivery of mail to some ISPs and mailbox providers.