Sender Policy Framework.

SPF is an anti-spam approach in which the Internet domain of an e-mail sender can be authenticated for that sender, thereby discouraging spam mailers, who routinely disguise the origin of their e-mail, a practice known as e-mail spoofing. SPF and other anti-spoofing initiatives, such as Domain Keys, work by making it easier for a mail server to determine when a message came from a domain other than the one claimed.