When you're setting up DMARC for your domain, a common question arises: what about subdomains? It's easy to assume that if you've configured DMARC for your main domain, example.com, then mail.example.com or marketing.example.com are automatically covered. While there's a degree of automatic inheritance, the full answer is a bit more nuanced.
By default, your DMARC policy for the organizational domain (also known as the root or top-level domain) typically applies to all its subdomains. This means if you have a DMARC record set up for yourdomain.com, its policy usually extends to sub.yourdomain.com unless specified otherwise. This is a convenient feature for many, ensuring broad protection without needing to create a separate record for every single subdomain you might use, or that might even exist without your direct knowledge.
However, the landscape of email sending often involves various platforms and services, each potentially using a different subdomain to send emails on your behalf. This is where the default inheritance might not always align with your specific needs. Understanding how this inheritance works and when to override it is key to maintaining optimal email deliverability and security.
The DMARC specification includes a tag specifically for subdomains called sp, which stands for subdomain policy. This tag allows you to define a DMARC policy that applies specifically to subdomains, even if it's different from the main domain's p (primary) policy. If the sp tag is present in your organizational DMARC record, it will dictate the policy for subdomains that do not have their own explicit DMARC record. You can read more about how the DMARC sp tag works in detail.
The key thing to remember is that an explicit DMARC record on a subdomain will always override any policy set by the parent domain, including the sp tag. So, if your example.com record has sp=quarantine, but marketing.example.com has its own DMARC record with p=reject, the marketing.example.com policy will be enforced for emails sent from that specific subdomain.
This hierarchy is crucial for granular control over your email ecosystem. If you're using various email service providers (ESPs) or different internal systems that send emails from distinct subdomains, you might want to tailor their DMARC policies. For instance, a transactional email subdomain might need a stricter policy than a marketing one during initial setup. This is why some platforms, like HubSpot, may specifically recommend setting up DMARC for your sending subdomains.
When to create explicit subdomain DMARC records
While default inheritance simplifies DMARC management, there are clear scenarios where you should consider setting up explicit DMARC records for your subdomains. This approach provides more precise control and can be beneficial for specific use cases.
Different policies: If you need a different DMARC policy (e.g., p=reject for transactional emails but p=none for marketing emails) for particular subdomains, an explicit record is necessary.
Third-party senders: When using third-party email providers for specific subdomains, they might require or recommend a dedicated DMARC record to ensure proper authentication and deliverability.
Granular reporting: Setting up a DMARC record for a subdomain allows you to receive separate aggregate (RUA) and forensic (RUF) reports, giving you a more detailed view of email authentication for that specific sending entity.
Addressing legacy systems: In some cases, older systems or tools might not correctly interpret the inherited DMARC policy from the parent domain. An explicit subdomain record can bypass these issues. You can also review Microsoft's guidance on DMARC setup steps for more context.
For example, if you use marketing.yourdomain.com for promotional emails and transactional.yourdomain.com for order confirmations, you might want a p=none policy on the marketing subdomain initially (to observe reports) and a p=reject on the transactional subdomain for maximum protection against spoofing once you are confident in its setup. To learn more, read our guide on DMARC setup best practices.
There’s no hard and fast rule that you must have separate DMARC records for subdomains, especially if your main domain's policy (or its sp tag) meets your security and deliverability requirements. However, if you find yourself needing more granular control or are instructed by a sending platform to configure DMARC at the subdomain level, it's a straightforward process.
Default DMARC vs. specific DMARC for subdomains
Default DMARC inheritance
Simplicity: One DMARC record for the root domain can cover all subdomains, reducing configuration overhead.
Broad protection: Ensures even undiscovered or unused subdomains are protected by a DMARC policy.
Consolidated reporting: DMARC reports for all subdomains are sent to the root domain's specified reporting address, centralizing data.
Specific DMARC record for a subdomain
Granular control: Apply different DMARC policies (e.g., p=none, p=quarantine, p=reject) to individual subdomains based on their sending behavior.
Targeted reporting: Receive separate DMARC reports for each subdomain, allowing for more focused analysis and troubleshooting.
Provider compliance: Fulfills specific DMARC requirements from email service providers (ESPs) that send emails from your subdomains.
Setting up a DMARC record for a subdomain
Setting up a DMARC record for a subdomain is very similar to setting it up for your main domain. You'll create a new TXT record in your DNS settings for that specific subdomain. The format remains largely the same, but the hostname will be different.
Choose your subdomain: Decide which subdomain requires its own DMARC policy, e.g., newsletters.yourdomain.com.
Construct the hostname: The hostname for the DMARC record will always begin with _dmarc. followed by your chosen subdomain. For our example, it would be _dmarc.newsletters.yourdomain.com.
Define the DMARC policy: Create the DMARC record string with your desired policy (e.g., p=none, p=quarantine, or p=reject) and reporting addresses. If you need help generating this record, our free DMARC record generator can assist.
Add to DNS: Publish this TXT record with your domain's DNS provider. Remember that it might take some time for DNS changes to propagate.
Here's an example of what such a record might look like in your DNS settings:
By following these steps, you gain granular control over the DMARC policy applied to specific subdomains, which is crucial for complex email infrastructures or when working with third-party sending services that require specific DMARC configurations.
The importance of monitoring
Once you've implemented DMARC for your subdomains, whether explicitly or implicitly, ongoing monitoring is essential. DMARC reports (RUA and RUF) provide invaluable insights into who is sending email on behalf of your domains and subdomains, and whether those emails are passing or failing authentication checks (SPF and DKIM). If you're managing multiple subdomains, this can quickly become complex.
Regularly analyzing these reports helps you identify legitimate sending sources that might not be correctly authenticated, as well as detect malicious activity like spoofing. Ignoring DMARC reports, particularly during the initial p=none phase, can lead to legitimate emails being quarantined or rejected once you move to stricter policies. If you're experiencing DMARC verification failures, the reports are your first line of defense in diagnosing the issue.
Monitor your DMARC reports
Whether you choose to rely on DMARC inheritance or implement explicit subdomain records, consistent monitoring is paramount. Keep an eye on your DMARC aggregate reports to ensure that all legitimate sending sources are properly authenticated. This vigilance helps prevent your emails from being blocked (or blacklisted) and maintains a strong sender reputation across all your domains and subdomains. Misconfigured DMARC can lead to your legitimate emails landing in spam, or even worse, being completely rejected.
It's also important to periodically review your subdomain usage. Some subdomains might be used for internal systems, testing environments, or forgotten marketing campaigns. Ensuring these also have appropriate DMARC policies, even if it's just inherited, closes potential security gaps. Staying on top of this ensures your email ecosystem remains secure and your deliverability high. You should also consider utilizing the 'sp' tag effectively for overall subdomain protection.
Views from the trenches
Best practices
Always start DMARC implementation with a p=none policy and monitor reports to identify all legitimate sending sources.
Utilize the 'sp' tag in your organizational DMARC record to set a default policy for subdomains without explicit records.
Publish explicit DMARC records for subdomains when you need a different policy or separate reporting for that specific sending entity.
Common pitfalls
Forgetting that a subdomain's explicit DMARC policy overrides the organizational 'sp' tag, leading to unexpected email handling.
Not monitoring DMARC reports, which can hide authentication failures or spoofing attempts on subdomains.
Assuming all subdomains are automatically protected at the desired level without checking their effective policies through reports.
Expert tips
For email service providers, often the most effective approach is to configure DMARC at the subdomain level.
In environments where DNS control is fragmented, DMARC at the subdomain level can provide localized protection.
The subdomain policy (p=) always takes precedence over the top-level domain's 'sp' tag if both exist.
Marketer view
Marketer from Email Geeks says that if DMARC is set up on the primary organizational domain, subdomains will inherit that policy by default unless a separate policy is specifically added to the subdomain.
2024-01-22 - Email Geeks
Marketer view
Marketer from Email Geeks says that setting up separate DMARC records for subdomains is only necessary if the policy for that subdomain needs to be different from the main domain's policy.
2024-01-22 - Email Geeks
Key takeaways
While DMARC automatically extends its protection to subdomains by default, the decision to set up explicit DMARC records for each subdomain hinges on your specific email sending strategy and security needs. The 'sp' tag provides a useful middle ground for applying a general subdomain policy from your organizational domain. However, for nuanced control, detailed reporting, or compliance with third-party sending services, dedicated subdomain DMARC records are the way to go.
The key is to understand the hierarchy: a specific subdomain's DMARC record will always override the parent domain's policy or 'sp' tag. Regardless of your chosen setup, continuous DMARC monitoring is critical to ensure proper authentication, prevent spoofing, and maintain strong email deliverability. This proactive approach ensures your email program is both secure and effective.
