|Anti-spam technique: Postage|
|Date of first use:||1980s|
|Difficulty of implementation:||Difficult to impossible|
|Where implemented:||MTA or gateway|
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.