Domains use public records (DNS) to route requests for various services (Web, e-mail, and so on) to the computers running those services. All domains already publish e-mail records (MX) to tell the world which computers receive e-mail for the domain. SPF works with domains that publish "reverse MX" records to tell the world which computers are allowed to send e-mail from the domain. When receiving a message from a domain, the recipient can check these records to ensure that the e-mail is from where it is supposed to come from. With SPF, these "reverse MX" records can be easily published: One line in the DNS is sufficient for this.