Sender policy framework. Defined in RFC 4408. Validates the HELO domain and the MAIL FROM address given as part of the SMTP protocol (RFC 2821 – the "envelope" layer). The MAIL FROM address is usually displayed as "Return-Path" if you select the "Show all headers" option in your email client. Domain owners publish records via DNS that describe their policy for which machines are authorized to use their domain in the HELO and MAIL FROM addresses, which are part of the SMTP protocol.