Code words

From ASRG

Anti-spam technique: Code words
Date of first use:	early 1990s
Effectiveness:	Medium
Popularity:	Low
Difficulty of implementation:	Low
Where implemented:	MUA or manual
Harm:	Low

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.