The Simple Mail Transfer Protocol permits any computer to send email claiming to be from any source address. This is exploited by spammers who often use forged email addresses, making it more difficult to trace a message back to its sender, 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 sender 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 require the use of TXT records,[2] 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".[3] The proposed standard, RFC 7208, says "Use of alternative DNS RR types was supported in SPF's experimental phase but has been discontinued".[2]
The sender address is transmitted at the beginning of the SMTP dialog. If the server rejects the sender, 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 sending address may be generated. If the server accepts the sender, 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 sender address. While the address in the Return-Path often matches other originator addresses in the mail header such as From or Sender, this is not necessarily the case, and SPF does not prevent forgery of these other addresses.
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.
The main benefit of SPF is to the owners of e-mail addresses that are forged in the Return-Path. They receive large amounts 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 a white list to identify known reliable senders. Scenarios like compromised systems and shared sending mailers limit this use.
