Sender Policy Framework (SPF) is a DNS-based email-authentication protocol that lets a domain specify which sending hosts are authorized to use its name in SMTP HELO/EHLO or MAIL FROM identities. Receiving systems can check the sending host against that policy. An SPF policy is published as a DNS TXT record beginning with v=spf1; SPF does not, by itself, authenticate the visible From address.
What SPF checks
SPF checks whether a sending host is authorized to use a domain in either the SMTP HELO/EHLO identity or the MAIL FROM identity. These are SMTP identities used during message delivery; they are distinct from the visible From header that a person typically sees in an email client. The IETF’s RFC 7208 defines SPF for those HELO and MAIL FROM identities.
That boundary matters: a successful SPF result is not proof that every identity associated with a message is authenticated, and SPF alone does not establish that the visible From address is genuine.
What an SPF record is and where it goes
An SPF record is a DNS TXT record published at the owner name for the domain to which the policy applies. Its version marker is v=spf1. A receiving system looks up the applicable policy and evaluates its terms against the sending host and SMTP identity.
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For illustration only, v=spf1 ... shows the required opening marker followed by policy content; the ellipsis is not valid SPF syntax and is not a record to publish. The correct policy content depends on which hosts the domain’s administrators intend to authorize.
Do not publish multiple SPF records that would be selected for the same owner name. RFC 7208 specifies that multiple such records are not permitted.
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How SPF evaluation works
SPF terms are evaluated in order. A mechanism can match or fail to match the sending host; its qualifier indicates the result when it matches:
+means pass.-means fail.~means softfail.?means neutral.
If no mechanism matches and the policy has no redirect modifier, the result is neutral. The outcome describes the SPF check for the relevant SMTP identity, not a verdict on the visible From header.
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The SPF DNS-lookup limit
RFC 7208 sets a limit of 10 DNS-causing terms per SPF evaluation. Terms such as include, a, mx, ptr, exists, and redirect count toward that limit. If evaluation exceeds 10, the result is permerror. This is a count of specified DNS-causing terms, not a rule that every individual DNS query is counted in the same way.
The standard also says implementations should limit void lookups to two; exceeding that limit produces permerror. This is a SHOULD recommendation, distinct from the 10-term limit.
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What SPF does—and does not—tell you
- SPF lets a domain publish which hosts are authorized to use its name in HELO/EHLO or MAIL FROM identities.
- It does not, by itself, authenticate the visible From header.
- A neutral, softfail, fail, or pass result is the result of SPF evaluation for the identity being checked; it should not be mistaken for a complete judgment about every identity in the message.
These boundaries follow the scope set out in RFC 7208, the IETF standard published in April 2014.
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