Sender Policy Framework (SPF) is a protocol designed to restrict who can use an organization's domain as the source of an email message. SPF blocks spammers and other attackers from sending email that appears to be from a legitimate organization
SMTP (Simple Mail Transfer Protocol) does not place any restrictions on the source address for emails, so SPF defines a process for the domain owners to identify which IP addresses are authorized to forward email for their domains.
The SPF protocol was published in 2014 by the Internet Engineering Task Force (IETF) in RFC 7208, "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1." SPF is an IETF Proposed Standard specification.