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SPF checks whether a sending host is authorized to use an SMTP identity; DKIM checks a cryptographic signature associated with a signing domain; and DMARC checks whether SPF or DKIM passes for a domain aligned with the visible From address, then publishes a handling preference and can request reports. They solve related but different problems: SPF and DKIM provide authentication results, while DMARC connects those results to the address recipients see.
SPF, DKIM, and DMARC at a glance
| Mechanism | Identity it checks | How it works | What the result provides |
|---|---|---|---|
| SPF | The domain in the SMTP MAIL FROM or HELO identity | The domain owner publishes DNS policy identifying authorized sending hosts; the receiver checks the sending host against that policy. | Host authorization for the checked SMTP identity. IETF RFC 7208 |
| DKIM | The signing domain named in the message signature | The message carries a cryptographic signature; the verifier retrieves the signing domain’s public key through DNS and verifies the signature. | A verifiable signing-domain assertion associated with the message. IETF RFC 6376 |
| DMARC | The domain in the RFC5322.From header, called the Author Domain | The receiver evaluates SPF and DKIM results and checks whether a successful identifier aligns with the Author Domain. The owner publishes a DNS policy and may request reports. | An aligned authentication result, a handling preference for failures, and optional reports. IETF RFC 9989 |
What does SPF check?
Sender Policy Framework (SPF) lets a domain owner state which hosts are authorized to use that domain in the SMTP MAIL FROM or HELO identity. The owner publishes SPF information in DNS, and receiving mail systems check whether the host that connected is authorized for the identity being evaluated. See RFC 7208.
In plain terms, SPF is a host-authorization check for an SMTP identity. It is not a cryptographic signature over the message, and an SPF pass does not, by itself, prove that the domain in the visible From header is authenticated. DMARC makes that additional connection through alignment.
What does DKIM check?
DomainKeys Identified Mail (DKIM) associates a message with a signing domain through a cryptographic signature. A verifier looks up the public key for that signing domain in DNS and uses it to check the signature. The signer may be the author’s organization, a mail relay, or another agent; the signing domain is not automatically the same as the author’s domain. RFC 6376 describes the mechanism.
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DKIM does not encrypt email. It verifies a signature over selected message content and headers. Changes to signed content in transit can invalidate that signature, although the mechanism is designed to support ordinary relaying when the signed material is not substantively changed.
What does DMARC add?
Domain-based Message Authentication, Reporting, and Conformance (DMARC) uses SPF and DKIM results and checks their relationship to the domain in the RFC5322.From header, also called the Author Domain. A message passes DMARC when at least one successful SPF or DKIM identifier is aligned with that Author Domain. A passing check for an unrelated domain is not enough.
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How alignment works
Under relaxed alignment, the authenticated domain and Author Domain share the same Organizational Domain. Under strict alignment, the domains must be identical. Thus a message can have a valid DKIM signature or pass SPF but still fail DMARC if the relevant authenticated domain does not align with the visible From domain. The current specification is RFC 9989, published in May 2026; it obsoletes RFC 7489 and RFC 9091.
Policy and reporting
A DMARC record in DNS communicates the domain owner’s handling preference for messages that fail DMARC validation and can request reports about use of the domain. Receivers use that policy as an input to their handling decisions; it does not guarantee identical treatment by every receiver or ensure inbox placement. RFC 9989 is the current specification for these behaviors: RFC 9989.
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SPF and DKIM authenticate different aspects of email. SPF asks whether a host is authorized to use an SMTP identity; DKIM asks whether a signature associated with a signing domain verifies. DMARC supplies the connection to the visible From domain, plus a failure-handling preference and optional reporting. It is not a third independent signature and does not replace SPF or DKIM.
For example, a message can carry a valid DKIM signature from a service that sends mail for many organizations. If that signing domain is not aligned with the message’s visible Author Domain, DKIM alone does not make the message pass DMARC. Alignment is the check that ties an SPF or DKIM result to the domain recipients see.
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Where authentication can fail or be misunderstood
- Forwarding and indirect mail flows: Forwarding can change the sending host SPF evaluates, and mailing lists or other intermediaries can modify message content covered by a DKIM signature. These flows can affect authentication outcomes; see RFC 7960 on DMARC interoperability with indirect email flows and RFC 6376 on DKIM signatures.
- A passing result is not a content verdict: SPF, DKIM, and DMARC concern domain use, signatures, and alignment. They do not establish that an email’s claims are truthful, that its content is safe, or that the recipient wants it.
- Authentication is not a universal deliverability guarantee: The standards define protocol behavior; they do not establish a universal percentage improvement in inbox placement, fraud prevention, or security.
Practical approach for domain owners
- Inventory legitimate senders. Identify the services and systems that send mail using your domain so that authorized services are accounted for in the relevant SPF and DKIM configuration.
- Check alignment with the visible From domain. Review whether each legitimate sending flow produces a successful SPF or DKIM identifier aligned with the RFC5322.From Author Domain.
- Use DMARC reports to observe outcomes. If your record requests reports, review them to understand which sources use the domain and how authentication and alignment are working.
- Adjust failure handling deliberately. After reviewing legitimate traffic and indirect flows, choose a DMARC policy that reflects your handling preference. Treat the policy as guidance to receivers, not as a guarantee of a particular outcome.
Which one should you think about first?
Use SPF when the question is whether a host is authorized for an SMTP MAIL FROM or HELO identity. Use DKIM when the question is whether a message’s signing-domain signature verifies. Use DMARC when the question is whether either result is tied to the visible From domain and what preference the domain owner publishes for failures. In administration, they are complementary parts of an email-authentication setup, not interchangeable alternatives.
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