Message validation techniques
As senders increasingly use whitelists, reputation, and introduction techniques to recognize mail from good senders, the bad senders have an increasing incentive to pretend to be good senders. Bad senders also engage in phishing, pretending to be a known sender with whom the recipient has a financial or other trust relationship, in order to steal the recipient's credentials.
Message validation techniques allow recipients to check whether an incoming message was really sent by the nominal sender. By themselves, they are not anti-spam techniques, since bad guys can and do validate their own mail, but they are an essential complement to recognition techniques.