Recognizing known correspondents

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Anti-spam technique: Recognizing known correspondents
Date of first use: early 1990s
Effectiveness: Medium
Popularity: High
Difficulty of implementation: Low
Where implemented: MUA
Harm: Low

Most mail programs provide an address book of some sort. A very popular technique is to whitelist mail from addresses in a user's address book, on the theory that if you know who they are, you probably want their mail.

This technique is moderately effective as a last pass after other spam filters. It requires that each user manually update his list of correspondents, leading to email newsletters that start with be sure to put our address into your address book.

Address books also tend to fill up with junk, particularly in programs that automatically add every outgoing To: address into the address book. This means that if you get an unwanted message, and respond to the message to tell them to stop, the address is now in your address book and implicitly whitelisted.