|Anti-spam technique: Code words|
|Date of first use:||early 1990s|
|Difficulty of implementation:||Low|
|Where implemented:||MUA or manual|
Code words are a variant on tagged addresses that put the tag in the Subject: line.
They're most commonly used informally by people with addresses that get a great deal of both legitimate mail and spam, who say "put <word> in the subject line when you write" and use a simple MUA filter that looks for the word.
In principle this could be done per-sender and automated the same way as address tags, but it appears that nobody has done so.