The Sender Policy Framework (SPF) is an open standard specifying a technical method to prevent sender address forgery. More precisely, the current version of SPF — called SPFv1 or SPF Classic — protects the envelope sender address, which is used for the delivery of messages. See the box on the right for a quick explanation of the different types of sender addresses in e-mails.

Even more precisely, SPFv1 allows the owner of a domain to specify their mail sending policy, e.g. which mail servers they use to send mail from their domain. The technology requires two sides to play together: (1) the domain owner publishes this information in an SPF record in the domain's DNS zone, and when someone else's mail server receives a message claiming to come from that domain, then (2) the receiving server can check whether the message complies with the domain's stated policy. If, e.g., the message comes from an unknown server, it can be considered a fake.