Spam filtering programs often use a combination of different types of spam filters. The three most common types are basic, challenge/response, and statistical.
Basic filters use a list of terms associated with know spam email messages.
Challenge/response filters send every message back to the sender who must verify that the sent message is not spam.
Statistical filters create and maintain a statistical profile of know spam messages.
To learn more about how each of these filters work, click the link below and view the animation.
Spam filter animation