When implementing DMARC, a common question arises regarding how it interacts with subdomains. Specifically, many wonder if DMARC policies and, crucially, DMARC reporting cascade down to subdomains or if separate records are always necessary for each. This can be a point of confusion, especially for organizations with complex domain structures.
Understanding the interplay between your organizational domain's DMARC record and its subdomains is vital for maintaining robust email security and deliverability. A misconfigured setup can lead to legitimate emails failing authentication, being blocked, or even allowing spoofed emails to pass through undetected.
This guide will clarify how DMARC policies and reporting behave across your domain hierarchy, providing insights into best practices for managing your email authentication effectively and ensuring you receive the comprehensive data you need.
How DMARC policies cascade to subdomains
DMARC policies set on your organizational domain, like example.com, do indeed cascade to all its subdomains, such as mail.example.com, by default. This means that if you define a p=quarantine policy for your main domain, it will apply to your subdomains unless otherwise specified. For more information on this cascading behavior, you can consult resources on how DMARC applies to subdomains.
The key mechanism for controlling subdomain policy is the sp tag within your DMARC record. This tag allows you to define a specific policy that applies only to subdomains, overriding the main p policy for the organizational domain. For instance, you might set a p=none for your root domain while setting sp=reject for all subdomains, offering greater control over specific sending practices. This is a common strategy to gradually implement stricter policies.
It's important to remember that a DMARC record published directly on a subdomain will always take precedence over any inherited policy from the organizational domain, including the sp tag. This hierarchy allows for fine-grained control, enabling you to define unique policies for specific subdomains that might have different sending requirements or risk profiles. You can learn more about how DMARC records on subdomains override root domain policies.
The DMARC sp tag is essential for managing subdomain policies. If your organizational domain has a DMARC record, the sp tag specifies the policy for all subdomains that do not have their own explicit DMARC record. If sp is omitted, subdomains inherit the main p policy.
Example DMARC record for root domain with subdomain policydns
Regarding DMARC reporting, the good news is that reporting generally does cascade to subdomains, even if they don't have their own explicit DMARC record. If a DMARC policy applies to a subdomain because of inheritance from the organizational domain (via the sp tag or default cascading), then aggregate (rua) and forensic (ruf) reports for that subdomain's email traffic should be sent to the addresses specified in the organizational domain's DMARC record.
This reporting mechanism is crucial for gaining comprehensive visibility into your entire email ecosystem. It allows you to monitor authentication results, identify potential spoofing attempts, and troubleshoot deliverability issues across all your sending domains, even if they're subdomains without individual DMARC records. For a deeper understanding, you can explore how DMARC policies and RUA/RUF settings inherit.
However, the way these reports are processed and displayed can vary significantly between DMARC reporting platforms. Some basic reporting engines might consolidate all subdomain data into the main domain's statistics by default, potentially obscuring specific insights. This could lead to the impression that you're not receiving detailed subdomain reports, even though the raw data is being sent. Advanced DMARC monitoring tools, like Suped, are designed to give you granular control and clear breakdowns for each subdomain, allowing you to differentiate between traffic originating from example.com and marketing.example.com.
DMARC policies cascade by default from the organizational domain to its subdomains.
The sp tag explicitly defines a policy for subdomains.
A direct DMARC record on a subdomain overrides any inherited policy.
DMARC reports (RUA/RUF) for subdomains are generally sent to the organizational domain's specified addresses.
Many reporting tools may group subdomain data by default, requiring specific features for segmentation.
Granular reporting for subdomains is best achieved with advanced DMARC monitoring platforms.
Monitoring and managing subdomain DMARC
Effective DMARC monitoring requires a holistic view of all your domains, including all subdomains, whether they have explicit DMARC records or inherit them. Without clear visibility into each subdomain's email traffic and authentication performance, you risk overlooking unauthorized sending sources or misconfigurations. This could significantly impact your email deliverability and expose your brand to spoofing threats.
Even if your subdomains inherit the main domain's DMARC policy, it is essential to ensure that their email streams are properly authenticated with SPF and DKIM. Monitoring these results through aggregate reports is critical for identifying and addressing any authentication failures. A unified platform that combines DMARC, SPF, and DKIM monitoring, alongside blocklist (or blacklist) and deliverability insights, provides the most comprehensive protection. This is where a tool like Suped becomes invaluable.
Tools that offer AI-powered recommendations can be particularly helpful for managing complex subdomain structures. These recommendations guide you on how to fix issues and strengthen your policy, making it easier to achieve a p=reject policy across all your sending domains. Suped’s real-time alerts ensure you are immediately aware of any authentication failures across your entire domain infrastructure, including subdomains, giving you peace of mind.
Unified view: Get a consolidated overview of all your domains and subdomains.
Granular insights: Easily drill down into specific subdomain performance data.
AI-powered recommendations: Actionable advice to optimize DMARC for each subdomain.
Real-time alerts: Stay informed about authentication issues across your entire email ecosystem.
MSP and multi-tenancy dashboard: Ideal for managing multiple client domains effectively.
Configuration
Policy Effect
Reporting Effect
Root domain DMARC with no sp
Policy applies to root domain and all subdomains by default.
Reports for root and subdomains sent to root rua address.
Root domain DMARC with sp
Policy for root domain, different policy for subdomains as specified by sp.
Reports for root and subdomains sent to root rua address.
Subdomain with its own DMARC record
Overrides root policy for that specific subdomain.
Reports for this subdomain sent to its own specified rua address.
Views from the trenches
Best practices
Always include the 'sp' tag in your organizational domain's DMARC record to explicitly define subdomain policy.
Regularly review DMARC aggregate reports to identify any unauthenticated traffic from subdomains.
Common pitfalls
Assuming DMARC reports automatically provide granular subdomain insights without a capable monitoring platform.
Neglecting to apply specific DMARC records to subdomains that have unique sending characteristics.
Expert tips
Even if DMARCbis is coming, current DMARC implementations do not 'tree walk' for policies.
Products typically roll subdomain stats into the main domain's data unless explicitly split out.
Expert view
DMARC policies cascade to subdomains, but they don't 'tree walk' through multiple levels of subdomains. For instance, if you have a DMARC record for 'a.b.c.example.com', it will check there and on the organizational domain, but not 'b.c.example.com'.
2024-09-17 - Email Geeks
Expert view
If your organizational domain applies a DMARC policy that cascades to a subdomain, you should still receive reports for that subdomain's pass or fail results.
2024-09-17 - Email Geeks
Understanding DMARC and your subdomains
In summary, DMARC policies do cascade to subdomains by default, and DMARC reporting for those subdomains also generally flows back to the rua and ruf addresses defined in your organizational domain's DMARC record. This means you should receive data about subdomain email authentication even without a specific DMARC record on each subdomain.
However, the key distinction often lies in how DMARC reporting tools interpret and present this data. While the reports are sent, a basic tool might consolidate all subdomain data, making it difficult to gain specific insights without more advanced features. This highlights the necessity of choosing a robust DMARC monitoring solution.
To truly secure your email sending and protect your brand across all domains and subdomains, continuous monitoring and actionable insights are essential. You need a platform that not only collects the reports but also helps you understand them and guides you on what steps to take. This proactive approach helps prevent spoofing, improves deliverability, and maintains a strong sender reputation.
Suped offers the most generous free plan available for DMARC reporting and monitoring, providing AI-powered recommendations, real-time alerts, and a unified platform for DMARC, SPF, and DKIM. Whether you're an SMB, a large enterprise, or an MSP, Suped makes DMARC accessible and actionable, giving you complete visibility and control over your entire email sending infrastructure.
