Which email authentication RFC number defines SPF?

Which email authentication RFC number defines SPF? RFC 7208 defines the current SPF specification.

RFC 7208, titled “Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1,” describes how a domain publishes the hosts authorized to send mail using that domain. Receiving systems look up the policy in DNS and compare it with the sending host, primarily for the SMTP envelope sender or HELO identity.

The document was published by the IETF in April 2014 and obsoleted RFC 4408, the earlier experimental specification. SPF policies are commonly published in DNS TXT records, using mechanisms such as ip4, ip6, mx, and include, plus qualifiers that produce results such as pass, softfail, or fail.

A frequent misunderstanding is that SPF authenticates the visible From header. By itself, it does not; it evaluates envelope identities. Domain-based Message Authentication, Reporting, and Conformance (DMARC) connects authentication results to the visible From domain, while DKIM uses a cryptographic signature. RFC 7208 is therefore not interchangeable with RFC 6376 for DKIM or RFC 7489 for DMARC.