Do subdomains need their own DMARC records if the main domain has one?

Key findings

• Default Inheritance: A DMARC record published at the organizational (parent) domain level generally applies to all its subdomains unless overridden.
• Explicit Records Override: Publishing a specific DMARC record for a subdomain (e.g., _dmarc.sub.example.com) will always take precedence over the parent domain's policy, including any sp tag.
• Subdomain Policy Tag: The sp tag in the main DMARC record can define a specific policy for all subdomains, distinct from the organizational domain's policy. Read more about the list of DMARC tags and their meanings.
• Authentication Strictness: Mailbox providers are increasingly strict, requiring proper DMARC authentication for all sending domains and subdomains to ensure messages reach the inbox.

Key considerations

• Policy Differentiation: Consider if subdomains have different sending purposes (e.g., transactional, marketing) that require unique DMARC policies (e.g., p=reject for one, p=none for another).
• Reporting Needs: You might want separate DMARC aggregate or forensic reports for specific subdomains to monitor their performance independently. Learn more about DMARC policies for organizational domains and subdomains.
• Avoiding Spam Folders: Missing DMARC records can lead to emails being marked as spam. As VerifyDMARC highlights, a DMARC record applied to a domain also affects subdomains unless a subdomain has its own DMARC record, which can be crucial for inbox placement. Ensure your emails are not going to spam.
• Simplicity vs. Control: While relying on inheritance simplifies setup, explicit subdomain records offer more granular control and can prevent unexpected deliverability issues, especially for high-volume or critical sending paths.

Key opinions

• DMARC Failures: Marketers often link DMARC failures on a specific subdomain directly to deliverability problems, such as emails landing in spam folders.
• Impact on Reputation: They believe that a domain's mail being routed to spam, even with DMARC, indicates a poor sender reputation that the authentication confirms, not causes.
• The Add it Anyway Approach: A common pragmatic approach is to add a DMARC record to a subdomain even if it's technically covered by the main domain, just to rule out potential parsing issues by ISPs.
• Microsoft Strictness: Many marketers acknowledge that Microsoft is particularly strict about DMARC authentication for all sending identities.

Key considerations

• Trust Your Instincts: If deliverability is suffering and a subdomain is unauthenticated, marketers often find that their initial instinct to add a specific DMARC record is correct, even when faced with contradictory advice.
• Comprehensive Troubleshooting: Addressing all authentication gaps, no matter how minor, is seen as part of a thorough troubleshooting process to eliminate potential causes of poor deliverability. This includes fixing email deliverability issues.
• DMARC's Role: DMARC (Domain-based Message Authentication, Reporting, and Conformance) confirms the authenticity of emails by checking SPF and DKIM. As WP Mail SMTP notes, you can only have one DMARC record per domain or subdomain to avoid confusion.
• Preventative Measures: Implementing DMARC on subdomains, even if not strictly required, can serve as a preventative measure against future deliverability problems. Consider the best practices for DMARC setup.

Key opinions

• Authentication is Key: Experts stress that proper DMARC authentication across all sending domains and subdomains is fundamental for earning and maintaining good sender reputation and deliverability.
• Conditional Inheritance: A DMARC record at the organizational domain can cover all subdomains, provided no subdomain requires a unique policy or separate reporting to a specific mailbox provider.
• Policy and Reporting Overrides: Subdomain-specific DMARC records are necessary if the policy or the Aggregate/Forensic reporting addresses (RUA/RUF) differ from the parent domain's settings.
• Diagnose DMARC Failures: If DMARC failures are occurring on a subdomain, it indicates a specific problem with that subdomain's authentication setup (even if the parent domain is compliant) that needs to be investigated.

Key considerations

• DMARC Alignment: For DMARC to pass, either SPF or DKIM must align with the From: domain. Ensure this alignment is correct for all sending subdomains. This is crucial for understanding DMARC, SPF, and DKIM.
• Granular Control vs. Defaults: While the default inheritance via the main domain's DMARC record and the sp tag offers some control, explicit subdomain records provide the most granular management over policies and reporting.
• ISP Interpretation: Some ISPs may have slightly different interpretations or preferences regarding inherited versus explicit DMARC records, making explicit records a safer bet for critical sending. NsLookup.io provides insight into using different DMARC records for subdomains.
• Preventing Blocklisting: Ensuring all sending subdomains are DMARC compliant helps prevent them from appearing on a blocklist (or blacklist), which can severely impact deliverability. You can proactively check for blocklist presence through blocklist checker tools.

Key findings

• Default Subdomain Policy: The DMARC specification dictates that if a specific DMARC record is not found for a subdomain, it will inherit the policy (p tag) of its organizational (parent) domain.
• Subdomain Policy Override (`sp` tag): The sp tag within an organizational domain's DMARC record allows the domain owner to specify a DMARC policy that applies only to subdomains, differing from the primary policy.
• Explicit Record Precedence: If a subdomain has its own DMARC record (e.g., published at _dmarc.sub.example.com), this record will override any inherited policy or the sp tag from the organizational domain.
• One Record Per Level: The DMARC standard permits only one DMARC record per specific domain or subdomain level. Publishing multiple records can lead to unpredictable behavior.

Key considerations

• Compliance with RFCs: Adhering to RFC 7489 ensures DMARC records are correctly interpreted by receiving mail servers, minimizing authentication failures.
• Strategic Use of sp: Leverage the sp tag to apply a uniform policy to all subdomains when a global override of the parent policy is desired, as discussed by DuoCircle in their explanation of the DMARC 'sp' tag.
• Granular Management: For subdomains with highly specific sending profiles or critical deliverability requirements, publishing an explicit DMARC record offers the most precise control. Suped provides guidance on how to set up DMARC records for subdomains.
• Avoid Redundancy and Conflicts: Carefully plan your DMARC deployment to avoid creating multiple DMARC records at the same domain level, which can lead to misconfiguration and authentication issues.