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What “strict” means in DMARC
DMARC evaluates mail against the domain in the message’s visible From: header (the Author Domain). A message passes DMARC when either SPF or DKIM passes and the authenticated domain aligns with that Author Domain.
Strict and relaxed alignment
| Alignment mode | What must match |
|---|---|
| Relaxed (default) | The authenticated domain and Author Domain may be different subdomains of the same Organizational Domain. |
| Strict | The authenticated domain must exactly equal the Author Domain. |
These modes apply separately to SPF and DKIM. They are configured with the DMARC tags aspf and adkim; each can be relaxed (r) or strict (s).
Policy enforcement is a separate choice
| Policy | Receiver instruction for DMARC failures |
|---|---|
p=none |
Monitor and deliver normally; useful while discovering senders and fixing authentication. |
p=quarantine |
Request that failures be treated as suspicious, commonly placed in spam or otherwise isolated. |
p=reject |
Request rejection of messages that fail DMARC. |
You can use strict alignment with p=none, or request p=reject while retaining relaxed alignment. Neither setting automatically enables the other.
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How to decide whether your domain is ready for p=reject
Readiness is evidence about your own domain, not a generic timetable. Use this sequence before changing the policy.
1. Inventory every legitimate sender
List every service that can send with your domain in the visible From address, including:
- Marketing and newsletter platforms
- Transactional applications and billing systems
- Customer-support and ticketing tools
- Monitoring, alerting and incident systems
- Cloud productivity services and contact forms
- Vendors that send on your behalf
Record each service’s envelope-from/SPF domain, DKIM signing domain, sending subdomains, and the From domains it uses. Include rarely used automated messages; an overlooked password-reset or invoice sender can be affected by rejection just as easily as a newsletter.
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2. Verify SPF and DKIM results and alignment
For each sender, inspect real messages and DMARC aggregate reports. A passing SPF result is insufficient if the SPF-authenticated domain does not align with the From domain. The same applies to DKIM: the signature must pass and its d= domain must align.
- Confirm that SPF includes every authorized service without exceeding SPF’s DNS-lookup limit.
- Confirm DKIM signing is enabled, keys are published, and selectors are current.
- Check the exact From domain, not merely the organization’s parent domain.
- Ensure at least one aligned mechanism (SPF or DKIM) works for every legitimate sending path.
3. Collect aggregate reports
Add a reporting address with the DMARC rua tag and analyze the resulting XML reports. Google’s “Set up DMARC” guidance documents the reporting address and DNS configuration. Reports should let you distinguish authorized traffic from spoofing and identify legitimate services that are failing alignment.
Do not treat a quiet report period as proof that no sender exists. Some systems send infrequently, and reports can be delayed or absent from individual receivers.
4. Test indirect mail flows
RFC 9989 warns that rejection can create interoperability problems for indirect flows. Forwarding services, mailing lists and intermediaries may alter the message or its authentication results. A message that passes when sent directly can fail after redistribution.
- Send test messages through major forwarding arrangements used by your staff and customers.
- Subscribe a test address to mailing lists that relay your mail, then inspect the delivered headers.
- Check whether list software rewrites the From address, applies ARC, or breaks DKIM signatures.
- Identify business workflows where recipients routinely forward messages.
Decide whether the protection gained from rejection outweighs the risk that a legitimate indirect message will be refused. DMARC policy is a request to receivers; behavior can vary by provider.
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5. Define the scope for subdomains
Decide whether the policy should cover subdomains. The sp tag sets a policy for subdomains when one is not published separately. A parent-domain p=reject does not mean every subdomain has identical sending configuration, so review transactional and delegated subdomains individually.
6. Stage enforcement deliberately
A cautious progression is:
- Use
p=nonewithruawhile you discover senders and correct SPF/DKIM alignment. - Move selected scope to
p=quarantineif you need to observe receiver handling of failures without requesting outright rejection. - Set
p=rejectonly after reports and controlled tests show that legitimate direct and indirect flows are covered. - Continue reviewing reports after enforcement; new vendors, DNS changes and product launches can introduce failures.
Apply changes to a carefully chosen domain or subdomain first when your DNS and mail architecture allow it. Keep a rollback plan so you can return to a less disruptive policy if legitimate traffic is affected.
Choosing strict alignment
Strict alignment can reduce ambiguity, but it is not a substitute for sender coverage. It requires the exact From domain to be used by the SPF-authenticated identity or DKIM signer.
When strict alignment fits
- You control the sending service and can configure a custom envelope-from and DKIM signing domain.
- Your organization intentionally separates brands or subdomains and wants exact-domain authentication.
- You have tested every sender that uses the From domain and can maintain those settings.
When relaxed alignment is safer
- A provider authenticates with a parent or sibling subdomain that you cannot change.
- Legacy applications or vendors support only organizational-domain alignment.
- Forwarding and list traffic is common and exact-domain preservation is unreliable.
Change adkim and aspf independently if necessary. Tightening one mechanism can expose a sender that previously relied on relaxed alignment even though the other mechanism still passes.
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Google’s current sender FAQ says bulk senders must set up both SPF and DKIM, while only one of them needs to align to satisfy Google’s sender-alignment requirement. Google recommends full alignment for both mechanisms. Its guidance also says DMARC alignment is not required for forwarded or mailing-list messages under that provider policy. This is a provider rule, not evidence that your domain is ready for p=reject.
Yahoo
Yahoo Sender Hub lists a valid DMARC policy of at least p=none among sender requirements. That minimum is not the same as enforcement and does not certify a domain for p=reject. Yahoo and Google requirements should not be treated as interchangeable.
Common mistakes when “getting strict”
- Equating strict alignment with rejection: They are separate settings with different effects.
- Checking only SPF or DKIM pass: DMARC also requires alignment with the visible From domain.
- Ignoring low-volume senders: Infrequent alerts and receipts still matter when rejection is enabled.
- Assuming direct delivery represents forwarding: Intermediaries can break authentication or alter headers.
- Using a provider minimum as a readiness test: A requirement such as
p=nonesays nothing about your complete sender inventory. - Publishing rejection without a rollback path: DNS changes can take time to propagate and failures may surface after deployment.
A practical readiness checklist
- Every legitimate sender is documented, including third-party and emergency systems.
- Each sender has a passing SPF or DKIM result with alignment to the From domain.
- SPF DNS lookups and DKIM key rotation are monitored.
- Aggregate reports are received at the configured
ruaaddress and reviewed. - Unknown sources have been investigated as spoofing, misconfiguration or forgotten services.
- Forwarding, mailing-list and other indirect paths have been tested or consciously accepted as a risk.
- Subdomain policy and delegated sending arrangements are documented.
- Owners know how to roll back the policy and contact affected vendors.
If any item is unresolved, remain at p=none for the affected scope or correct the gap before requesting rejection. No standards document or provider requirement can determine readiness for a particular domain without this operational evidence.
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