Taxonomy of anti-spam techniques

Many people have invented many anti-spam techniques over the past decade. And a lot of the techniques keep being reinvented. Our goal here is to list all the anti-spam techniques we know, both the good ones and the bad ones.

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Message content techniques

• Static filters

• Adaptive filters

• URL filtering

SMTP techniques

• Timing and protocol defects

• Greylisting

• Callbacks

• Rate limits

Address management

• Tagged addresses

• Code words

• Disposable addresses

Network techniques

• DNS blacklists

• DNS validation

• HELO/EHLO Pattern Matching

• Tarpitting and other timing techniques

Whitelist techniques

• Recognizing known correspondents

• Path validation

• Signatures

• Subject keywords

Message validation techniques

• Cryptographic signatures

• Signature callback

Introduction techniques

• Challenges

• Proof of work

• Postage

• Attention bonds

• Captchas

Administrative techniques

• No spam notices

• Do not spam lists