Summary
DMARC policies primarily apply to the organizational domain. Subdomains inherit this policy by default unless a specific subdomain policy (the sp tag) is defined or a distinct DMARC record exists for the subdomain. CNAME records introduce complexities because a CNAME cannot coexist with other records, like a TXT record for DMARC, on the same hostname.
Key findings
- Inheritance: DMARC policies set on a main domain generally apply to its subdomains by default.
- Explicit subdomain policy: The sp tag in a DMARC record allows defining a specific policy for subdomains, overriding the main p policy.
- CNAME conflict: A CNAME record cannot exist at the same DNS label as other record types, including the _dmarc TXT record. This is a fundamental DNS rule. For more details on this, refer to discussions on CNAME and TXT record coexistence.
- DMARC record location: DMARC policies are always looked up as TXT records at _dmarc.yourdomain.com or _dmarc.yoursubdomain.yourdomain.com.
Key considerations
- DNS rule adherence: Ensure your DNS setup for DMARC (specifically the _dmarc entry) adheres to fundamental DNS rules, avoiding CNAMEs where a TXT record is expected for the DMARC policy itself.
- Consolidated management: CNAMEs can be used to point subdomain DMARC records (e.g., _dmarc.sub.domain.com CNAME _dmarc.main.domain.com) to a central DMARC record, allowing for easier management of multiple subdomains.
- Policy application: Clearly define if you want subdomains to inherit the main domain's DMARC policy or have their own, as detailed in our guide on DMARC policies for organizational domains and subdomains.
- Testing changes: Always test DNS changes thoroughly to prevent email deliverability issues.
What email marketers say
Email marketers often encounter confusion regarding DMARC implementation, especially when dealing with subdomains and the interplay with CNAME records. The primary concern is ensuring consistent policy application and avoiding unexpected email authentication failures. While the default inheritance simplifies some setups, custom configurations require careful attention to DNS best practices.
Key opinions
- Default inheritance: Many marketers assume the organizational DMARC policy will apply to subdomains without explicit configuration.
- Confusion with tools: Some diagnostic tools might present misleading information when encountering non-standard or conflicting DNS configurations, such as a CNAME at the _dmarc label.
- Simplifying management: There is a desire to use DNS mechanisms like CNAMEs to centralize DMARC management for numerous subdomains.
- Importance of `sp` tag: Marketers recognize the sp tag as a crucial element for granular control over subdomain DMARC policies.
Key considerations
- Verify tool output: Do not solely rely on automated tool messages, cross-reference with DMARC specifications and basic DNS rules.
- Subdomain strategy: Decide on a clear DMARC strategy for subdomains, whether to inherit, override, or configure specific policies per subdomain.
- Impact on deliverability: Incorrect DMARC setup, especially with CNAMEs, can lead to authentication failures and impact inbox placement. For more information on how email servers handle authentication failures, consider checking relevant resources.
- DNS hierarchy: Understand how DNS lookups for DMARC records traverse the domain hierarchy to ensure policies are applied as intended.
Marketer view
Marketer from Email Geeks observes unusual DMARC messages for a domain without explicit subdomain settings, indicating that the diagnostic tool shows something unexpected.
Marketer view
Marketer from Email Geeks notes inconsistencies in DMARC checks across similar domains, highlighting that other domains without subdomains do not display similar warning messages.
What the experts say
Email deliverability experts emphasize that DMARC's interaction with subdomains and CNAMEs is often a source of confusion due to the intricacies of DNS. They stress the importance of adhering to DNS standards and understanding DMARC's policy inheritance model to ensure proper email authentication and avoid inadvertent blocklisting.
Key opinions
- DNS fundamentals: Experts underline that DMARC primarily relies on TXT records and does not inherently 'follow' CNAMEs for policy lookups at the _dmarc label itself, due to DNS limitations.
- Subdomain policy (`sp`): The sp tag is the intended mechanism for defining specific DMARC policies for subdomains, distinct from the organizational domain's policy.
- Wildcard CNAMEs: Experts caution against the misuse of wildcard CNAMEs, particularly when trying to apply them in ways that conflict with DMARC's specific record lookup mechanism.
- Inheritance vs. explicit: Understanding whether a subdomain inherits the organizational policy or has its own explicit record is critical for correct DMARC application.
Key considerations
- Avoid CNAME conflicts: Never place a CNAME record at the _dmarc label if you intend to publish a DMARC TXT record there, as this violates DNS rules.
- Centralized DMARC management: Use CNAMEs for DMARC as _dmarc.sub.domain CNAME _dmarc.org.domain to centralize policies for many subdomains, but confirm this is supported by your DNS provider and receiving servers.
- Comprehensive testing: Always test DMARC configurations rigorously across all sending domains and subdomains, ensuring adherence to DNS standards.
- Policy progression: Consider a phased approach for DMARC policy deployment, starting with p=none to gather data before moving to p=quarantine or p=reject. For more on DMARC's fundamental role, you can explore resources like Twilio's explanation of DMARC.
Expert view
Expert from Email Geeks suggests that in the absence of a specific subdomain policy, the domain-wide DMARC policy will naturally apply to subdomains, advising to contact support for further clarification on specific tool messages.
Expert view
Expert from Email Geeks clarifies that if a parent domain has a DMARC record with p=none and no additional subdomain policy (sp=) or individual DMARC record for the subdomain, then the p=none from the parent domain will be applied.
What the documentation says
Official DMARC documentation and related RFCs provide the foundational rules for how DMARC policies are applied, particularly concerning subdomains and the specific lookup mechanisms for DMARC records. These documents clarify that policy inheritance is the default behavior and outline the use of the sp tag for explicit subdomain policies.
Key findings
- Default policy application: DMARC policies are designed to apply to the organizational domain and its subdomains unless explicitly overridden.
- Sp tag's role: The sp tag (subdomain policy) allows a domain owner to specify a different DMARC policy for subdomains than the one defined for the organizational domain.
- DNS record type: DMARC policies are published as TXT records within the DNS, typically at the _dmarc subdomain label.
- No CNAME at DMARC label: Standard DNS rules dictate that a CNAME record must not coexist with any other record types at the same label, which applies to the _dmarc TXT record location.
Key considerations
- RFC adherence: Implement DMARC policies strictly according to RFCs to ensure universal acceptance and correct interpretation by receiving mail servers.
- Hierarchy of policy: Understand the precedence of policies (explicit subdomain DMARC record > sp tag > organizational p tag).
- Impact on DNS resolution: Recognize that CNAMEs, while useful for other DNS purposes, have specific limitations when used in conjunction with DMARC TXT records.
- Brand protection: Using DMARC with proper subdomain policies helps protect all subdomains from unauthorized use, even those not actively sending email. DMARC allows specific subdomain configurations when policies cannot be merged, offering flexibility.
Technical article
Documentation from NsLookup.io states DMARC permits only one DMARC record per domain, but subdomains can be used when DMARC policies cannot be merged, providing a way to handle complex domain structures.
Technical article
Documentation from HostAdvice confirms that the DMARC policy set for the primary domain applies to its subdomains by default, but advises setting a specific policy for a subdomain if desired, for more granular control.
Related resources
8 resources
Related pages
DMARC policies for organizational domains and subdomains explained
What is the best practice for DMARC record placement for subdomains?
When should you use DMARC p=none, p=quarantine, or p=reject policies?
How to implement DMARC p=reject policy safely to avoid email deliverability issues?
How to troubleshoot SPF authentication issues with multiple ESPs, subdomains, and CNAMEs?
A simple guide to DMARC, SPF, and DKIM
List of DMARC tags and their meanings
How to fix common DMARC issues in Microsoft 365 and Google Workspace
How does DMARC work, why implement it with SPF and DKIM, and what tools are available for DMARC record creation?
How to safely transition your DMARC policy to quarantine or reject
