Sender Policy Framework (SPF) is an email authentication method that allows checking whether the sending mail server is authorized to originate mail from the email sender's domain. This authentication only applies to the email sender listed in the "envelope from" field during the initial SMTP connection. If the email is bounced, a message is sent to this address, and for downstream transmission it typically appears in the "Return-Path" header. To authenticate the email address which is actually visible to recipients on the "From:" line, other technologies, such as DMARC, must be used. Forgery of this address is known as email spoofing, and is often used in phishing and email spam. The list of authorized sending hosts and IP addresses for a domain is published in the DNS records for that domain. Sender Policy Framework is defined in RFC 7208 dated April 2014 as a "proposed standard".
History The first public mention of the concept was in 2000 but went mostly unnoticed. No mention was made of the concept again until a first attempt at an SPF-like specification was published in 2002 on the IETF "namedroppers" mailing list by Dana Valerie Reese, who was unaware of the 2000 mention of the idea. The very next day, Paul Vixie posted his own SPF-like specification on the same list. These posts ignited a lot of interest, led to the forming of the IETF Anti-Spam Research Group (ASRG) and their mailing list, where the SPF idea was further developed. Among the proposals submitted to the ASRG were "Reverse MX" (RMX) by Hadmut Danisch, and "Designated Mailer Protocol" (DMP) by Gordon Fecyk. In June 2003, Meng Weng Wong merged the RMX and DMP specifications and solicited suggestions from others. Over the next six months, a large number of changes were made and a large community had started working on SPF. Originally SPF stood for Sender Permitted From and was sometimes also called SMTP+SPF; but its name was changed to Sender Policy Framework in February 2004. In early 2004, the IETF created the MARID working group and tried to use SPF and Microsoft's CallerID proposal as the basis for what is now known as Sender ID; but this collapsed due to technical and licensing conflicts. The SPF community returned to the original "classic" version of SPF. In July 2005, this version of the specification was approved by the IESG as an IETF experiment, inviting the community to observe SPF during the two years following publication. On April 28, 2006, the SPF RFC was published as experimental RFC 4408. On April 25, 2014 IETF published SPF in RFC 7208 as a "proposed standard" which "obsoleted" RFC 4408 and, as of May 2026, RFC 7208 had not been obsoleted.
Principles of operation
The Simple Mail Transfer Protocol permits any computer to send email claiming to be from any source address. This is exploited by spammers and scammers who often use forged email addresses, making it more difficult to trace a message back to its source, and easy for spammers to hide their identity in order to avoid responsibility. It is also used in phishing techniques, where users can be duped into disclosing private information in response to an email purportedly sent by an organization such as a bank. SPF allows the owner of an Internet domain to specify which computers are authorized to send mail with envelope-from addresses in that domain, using Domain Name System (DNS) records. Receivers verifying the SPF information in TXT records may reject messages from unauthorized sources before receiving the body of the message. Thus, the principles of operation are similar to those of DNS-based blackhole lists (DNSBL), except that SPF uses the authority delegation scheme of the Domain Name System. Current practice requires the use of TXT records, just as early implementations did. For a while a new record type (SPF, type 99) was registered and made available in common DNS software. Use of TXT records for SPF was intended as a transitional mechanism at the time. The experimental RFC, RFC 4408, section 3.1.1, suggested "an SPF-compliant domain name SHOULD have SPF records of both RR types". The proposed standard, RFC 7208, says "use of alternative DNS RR types was supported in SPF's experimental phase but has been discontinued". The envelope-from address is transmitted at the beginning of the SMTP dialog. If the server rejects the domain, the unauthorized client should receive a rejection message, and if that client was a relaying message transfer agent (MTA), a bounce message to the original envelope-from address may be generated. If the server accepts the domain, and subsequently also accepts the recipients and the body of the message, it should insert a Return-Path field in the message header in order to save the envelope-from address. While the address in the Return-Path often matches other originator addresses in the mail header such as the header-from, this is not necessarily the case, and SPF does not prevent forgery of these other addresses such as sender header. Spammers can send email with an SPF PASS result if they have an account in a domain with a sender policy, or abuse a compromised system in this domain. However, doing so makes the spammer easier to trace. SPF benefits owners of email addresses that are forged in the Return-Path. They receive large numbers of unsolicited error messages and other auto-replies. If such receivers use SPF to specify their legitimate source IP addresses and indicate FAIL result for all other addresses, receivers checking SPF can reject forgeries, thus reducing or eliminating the amount of backscatter. SPF has potential advantages beyond helping identify unwanted mail. In particular, if a sender provides SPF information, then receivers can use SPF PASS results in combination with an allow list to identify known reliable senders. Scenarios like compromised systems and shared sending mailers limit this use.
Reasons to implement If a domain publishes an SPF record, spammers and phishers are less likely to forge emails pretending to be from that domain, because the forged emails are more likely to be caught in spam filters which check the SPF record. Therefore, an SPF-protected domain is less attractive to spammers and phishers. Because an SPF-protected domain is less attractive as a spoofed address, it is less likely to be denylisted by spam filters and so ultimately the legitimate email from the domain is more likely to get through.
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