Subject keywords

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Anti-spam technique: Subject keywords
Date of first use: late 1990s
Effectiveness: Low to medium
Popularity: Low
Difficulty of implementation: Medium
Where implemented: MUA
Harm: Low to medium

Correspondents put a particular word in the subject line of mail. Then the recipient sets up a mail filter to discard or sideline mail without that word.

For individual correspondents, it's rarely effective since there are few people whose correspondents are willing to jump through hoops to send them mail. It has been somewhat effective on submission addresses for mailing lists and newsgroups.

It wouldn't be hard to adjust MUA address books to include a subject keyword, but as far as we know nobody's done that.