|Anti-spam technique: Subject keywords|
|Date of first use:||late 1990s|
|Effectiveness:||Low to medium|
|Difficulty of implementation:||Medium|
|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.