Tagged addresses

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Anti-spam technique: Tagged addresses
Date of first use: early 1990s
Effectiveness: High
Popularity: Medium
Difficulty of implementation: Medium
Where implemented: MTA or MUA
Harm: Low

Tagged addresses add a second part to an existing address that is not used for mail routing, but can be used for sorting or filtering mail. The exact syntax varies by implementation. Sendmail uses user+tag@dom.ain while most other systems use user-tag@dom.ain. In a few cases the user name is moved into the domain part to deter tagging as tag@user.dom.ain.

Tagged addresses are different from disposable addresses in that each tagged address is assigned to a specific correspondent or role. Typically a user will use a different tag for each mailing list to which he subscribes, and each web site that demands signup information. If a message arrives to a tagged address and the sender is the party to whom the tag was given, the message is presumably legitimate. If a tagged message arrives from someone else, it's probably spam and the tag tells you who leaked it.

ZoEmail was a commercial tagged mail system, dating from Robert J. Hall's 1996 work. They provided addresses such as user.tag@zoemail.net. A similar system is TrashMail, which provides addresses like whatever@trashmail.net. TrashMail distribute a PHP script that implements a protocol to register new tagged addresses, and a FireFox add-on featuring two-click paste of an address created and registered on-the-fly directly into into the target form field. The adjustable options are the number of messages and the time span that the new address will last for. Notice how the concept of tagged addresses may conflate with that of disposable addresses when used that way.