"SPF is an open standard specifying a technical method to prevent sender address forgery.
"SPF is the protocol-level identification of the delivering mail server, and it is usually invisible to recipients. It is mirrored in the Return-Path header, the address to which mail delivery errors (or bounces) are sent. For individual e-mail addresses or small domains, it may sometimes be set to the user's e-mail address. But for larger and more professionally managed domains, it is usually a domain related to the mail server that sent the message.
"SPF protects the envelope sender address, which is used for the delivery of messages. This allows the owner of a domain to specify its mail-sending policy by specifying which mail servers are used to send mail from the domain. The technology requires two sides to participate: 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, the receiving server can check whether the message complies with the domain's stated policy. If the message comes from an unknown server, it can be considered a fake."
