Challenge-response spam filtering

Internet Security. A type of spam filter that replies with a challenge to the given sender. This spam filter verifies the sending email address.

The challenge sends a SMTP RCPT TO command with the given sender's email address. This email address comes from the return-path in the email envelope header. The correct response from a sender's email server would be 250 Ok or 2550 2.1.5 OK. A failed response would be 550 <message>. The recipient server accepts an email message from a sender that passes this challenge.

This isn't the same as challenge-response authentication.

To validate the content of an email message body, a system might use a Naive Bayesian filter.