Static subject keyword

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Anti-spam technique: Static subject keyword
Date of first use: early 1990s
Effectiveness: Low
Popularity: High, built into many MUAs
Difficulty of implementation: Easy
Where implemented: MUA or MTA
Harm: Variable

Subject filters are popular, since large amounts of spam still is about things that rarely appears in legitimate mail, such as certain drugs, body parts, and implausible third-world military and financial stories.

They are easily subverted by euphemism or deliberate misspellings, e.g. v1@gra.

Naive filters tend to get unexpected false positives. For example a filter for the drug name 'cialis' would match the common word 'specialist'. While it's easy to fix this specific case by checking for word boundaries, there are many words such as 'breast' that occur in multiple contexts, some wanted, some not.