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What an SPF record does
A domain administrator publishes an SPF policy in DNS. When a receiving mail system evaluates a message, it checks whether the sending host is authorized for the relevant SMTP identity. The IETF describes an SPF record as a DNS record declaring which hosts are or are not authorized to use a domain name for the “HELO” and “MAIL FROM” identities (RFC 7208, April 2014).
Which email identities SPF checks
SPF’s defined checks apply to the domain used in the SMTP HELO/EHLO greeting or the MAIL FROM command. These are part of the message’s SMTP transaction; they are not the same thing as the visible From header that a person typically sees in an email client. A passing SPF result therefore does not, on its own, prove that the visible From address is authorized.
Where the record is published
The SPF policy belongs in a DNS TXT record at the owner name for the domain it applies to. Its version marker is v=spf1. A domain should not publish multiple SPF records that would cause multiple records to be selected for that same owner name; SPF evaluation does not treat multiple policies there as a combined list.
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How SPF evaluation works
SPF mechanisms are evaluated in order. A mechanism that matches can return a result determined by its qualifier:
+means pass.-means fail.~means softfail.?means neutral.
If no mechanism matches and the record has no redirect modifier, the result is neutral. These terms describe SPF evaluation outcomes; they are not a complete judgment about whether a message is trustworthy.
The DNS lookup limit
RFC 7208 limits an SPF evaluation to 10 DNS-causing terms. Terms such as include, a, mx, ptr, exists, and redirect count toward that limit. If an evaluation exceeds it, the result is permerror. This is a limit on DNS-causing terms in an SPF evaluation, not a claim that each DNS query or query type counts identically.
The RFC also says SPF implementations should limit void lookups to two; exceeding that recommended limit produces permerror. This is a “SHOULD” recommendation in the standard, distinct from the 10-term limit.
SPF’s place in email authentication
SPF answers a specific authorization question about a sending host and an SMTP identity. It does not authenticate every identity associated with a message, and a successful SPF check does not independently validate the visible From header. Treat an SPF result as one defined piece of email authentication, not a universal guarantee about message origin.
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