How can I resolve DMARC verification failures when using a subdomain for email sending?
Matthew Whittaker
Co-founder & CTO, Suped
Published 16 Jul 2025
Updated 17 Aug 2025
8 min read
Encountering DMARC verification failures when sending emails from a subdomain can be a frustrating experience. You might see bounce messages like "550 5.7.509. Access denied, sending domain does not pass DMARC verification and has a DMARC policy of reject." This indicates that the recipient's mail server is rejecting your emails because your domain's DMARC policy, which is set to 'reject', wasn't satisfied by the authentication checks. It's a common issue, but it often points to a gap in your email authentication setup specifically for the subdomain.
While your main domain might have DMARC, SPF, and DKIM configured correctly, subdomains require careful attention because they can either inherit the main domain's policies or need their own explicit DNS records. When these are misconfigured or missing, it leads to authentication failures, resulting in your legitimate emails being blocked or marked as spam. Let's explore how to diagnose and resolve these issues.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is an email authentication protocol that builds on SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail). Its primary role is to tell receiving mail servers what to do with emails that fail SPF or DKIM authentication for your domain. This helps prevent email spoofing and phishing attacks by ensuring only authorized senders are using your domain.
When you use a subdomain, such as marketing.yourdomain.com, for email sending, DMARC alignment becomes crucial. DMARC requires that the domain in the "From" header (the visible sender) aligns with the domain verified by SPF or DKIM. This alignment can be either strict or relaxed. For subdomains, a DMARC record at the organizational domain level can apply to all subdomains, or a specific DMARC record can be published for the subdomain itself.
Without proper configuration, even if your main domain passes DMARC, emails from your subdomain might fail. It's a common misconception that simply having a DMARC record for the root domain is sufficient for all subdomains, but this isn't always the case, especially if your sending practices or third-party email service providers (ESPs) introduce complexities. Understanding how this all works is vital for improving your email deliverability.
Understanding the core protocols
DMARC: A standard that uses SPF and DKIM to authenticate email and defines policies for unauthenticated mail. It also provides reporting capabilities.
SPF: A DNS TXT record that lists mail servers authorized to send emails on behalf of your domain, preventing spoofing of your domain in the envelope MAIL FROM address.
DKIM: Adds a digital signature to outgoing emails, allowing the recipient's server to verify that the email hasn't been tampered with and truly originated from the claimed sender. This uses your email service provider's domain as the signing domain.
Diagnosing subdomain DMARC failures
The most frequent cause of DMARC failures for subdomains is a mismatch or absence of SPF and DKIM records specifically for that subdomain. Even if your root domain has perfect records, the subdomain needs its own set of authenticated sending permissions, or it needs to properly inherit them. When an email leaves your system via a third-party sending service, the MAIL FROM domain or the DKIM signing domain might not align with your subdomain's "From" header, causing DMARC to fail.
Another significant factor is DMARC alignment. DMARC checks whether the domain in the "From" header (the human-readable sender) aligns with the domain validated by SPF (the MAIL FROM domain) or DKIM (the d= tag in the signature). If you're using a third-party email service, they might be signing with their own domain or a different subdomain, leading to an alignment failure.
Email forwarding can also disrupt SPF and DKIM authentication, leading to DMARC failures. When an email is forwarded, the original sender's SPF record often breaks because the forwarding server's IP address isn't authorized in the original SPF record. While DKIM is more resilient to forwarding, certain forwarding mechanisms can still break the signature. If you're experiencing DMARC failures despite correct SPF/DKIM, forwarded emails could be the culprit.
Strict alignment (as=s)
This policy requires an exact match between the domain in the "From" header and the domain validated by SPF or DKIM. For example, if your "From" address is info@marketing.yourdomain.com, then SPF and DKIM must also authenticate marketing.yourdomain.com exactly.
Impact
While more secure, strict alignment can lead to DMARC failures if not all sending systems align perfectly with your subdomain, especially when using third-party services that might send from their own subdomains.
Relaxed alignment (as=r)
This policy allows for subdomains. If your "From" address is info@marketing.yourdomain.com, SPF or DKIM can authenticate marketing.yourdomain.com or yourdomain.com. This is often the default or recommended setting for flexibility.
Impact
Relaxed alignment is generally easier to implement and causes fewer DMARC failures, especially when dealing with various third-party email providers. It provides a good balance between security and deliverability.
Practical steps to resolve DMARC failures
The first step in resolving DMARC verification failures is to gather information. DMARC reports (aggregate and forensic) are invaluable for this, as they provide data on who is sending email on behalf of your domain, and whether those emails are passing or failing authentication. By diagnosing DMARC failures using these reports, you can pinpoint the source of the problem.
Next, verify the SPF and DKIM records for your specific subdomain. For SPF, ensure that all IP addresses and domains authorized to send emails from your subdomain are included. For DKIM, confirm that the DKIM record is published at the correct selector for your subdomain and that your email service provider is signing emails with the correct domain. Sometimes, a third-party sender might use a different signing domain than your subdomain, requiring a relaxed DMARC alignment.
If you don't have a separate DMARC record for your subdomain, it will likely inherit the DMARC policy from your main domain. If your main domain's DMARC policy is set to p=reject or p=quarantine, misconfigured subdomain emails will be rejected or quarantined. A simple initial fix for testing is to set up a DMARC record for the subdomain with a policy of p=none. This will allow emails to be delivered while you troubleshoot.
Remember, DMARC requires at least one of SPF or DKIM to pass authentication and alignment. If only one fails, DMARC might still pass if the other is correctly configured and aligned. For instance, if SPF fails due to forwarding but DKIM is strong and aligned, DMARC could still pass. Focus on ensuring both SPF and DKIM are set up correctly and achieve the necessary alignment for your subdomain sending.
Advanced considerations and best practices
Once you've resolved the immediate DMARC failures, the work isn't over. DMARC is an ongoing process that requires continuous monitoring and gradual policy enforcement. Start with a p=none policy to collect reports without impacting deliverability. Analyze these reports to identify all legitimate sending sources for your subdomain and ensure they are properly authenticated. This data will help you transition to stronger policies like p=quarantine and eventually p=reject.
Consider using separate subdomains for different types of email sending, such as marketing emails, transactional emails, or internal communications. This can help isolate reputation issues. If one subdomain gets listed on a blacklist or blocklist (or blocklist), it won't necessarily affect your other sending streams. This strategy requires diligent setup of SPF, DKIM, and DMARC for each subdomain, but it offers better control and risk management for your overall email program.
Engaging with your email service provider is also crucial. They often have specific requirements or recommendations for configuring SPF, DKIM, and DMARC when using their platform, especially for subdomains. For instance, services like Amazon SES or SendGrid provide guides for DMARC compliance. Ensure their sending infrastructure is authorized via your SPF and that their DKIM signing aligns with your DMARC policy. This proactive approach helps in boosting email deliverability rates.
Policy (p=)
Description
Impact on deliverability
none
Instructs receiving servers to take no action on emails that fail DMARC. Used for monitoring and collecting reports.
No direct negative impact on deliverability, but offers no protection against spoofing. Ideal for initial deployment.
quarantine
Suggests receiving servers move emails that fail DMARC to the spam folder or quarantine them.
Reduces deliverability for non-compliant emails. Use after analyzing DMARC reports from Google and Yahoo.
reject
Instructs receiving servers to reject emails that fail DMARC verification entirely.
Strongest protection against spoofing but can severely impact deliverability if legitimate emails fail DMARC.
Finally, regularly review your DMARC reports. Tools for DMARC monitoring provide clear insights into your email ecosystem, helping you spot any unauthorized sending or legitimate sources that are failing authentication. This proactive approach is key to maintaining a healthy sender reputation and avoiding DMARC-related delivery issues in the long run.
Views from the trenches
Best practices
Always analyze DMARC reports thoroughly to identify all legitimate sending sources for your domain and subdomains.
Implement SPF and DKIM authentication for all email-sending subdomains, ensuring proper alignment.
Use a p=none DMARC policy initially for subdomains to gather data without blocking emails.
Gradually transition DMARC policies from p=none to quarantine, then to reject, as confidence grows.
Consult your email service provider's documentation for specific DNS configuration requirements.
Common pitfalls
Forgetting to set up SPF or DKIM records for new subdomains used for email sending.
Assuming root domain DMARC policy automatically covers all subdomain sending scenarios without explicit checks.
Immediately setting a p=reject policy for subdomains, leading to legitimate emails being blocked.
Not monitoring DMARC reports, thus missing vital information about authentication failures.
Ignoring the impact of email forwarding on SPF authentication.
Expert tips
For complex setups with multiple ESPs, using relaxed SPF and DKIM alignment can reduce authentication failures.
Implement a DMARC record for each subdomain even if it inherits the parent's policy, to explicitly control its behavior.
Regularly check your DMARC aggregate and forensic reports to detect new sending sources or configuration issues.
When troubleshooting, focus on the DMARC record's alignment with both SPF and DKIM for each message.
If SPF fails due to forwarding, ensure DKIM is strong and aligned, as DMARC only requires one to pass.
Expert view
Expert from Email Geeks says that DMARC failures can often be resolved by utilizing online tools that provide comprehensive analysis of DNS entries and DMARC alignment.
2023-11-09 - Email Geeks
Marketer view
Marketer from Email Geeks says that when setting up a new subdomain for email, ensure that all messages from that subdomain originate from authorized systems as configured in your SPF and DKIM records.
2023-11-09 - Email Geeks
Summary of resolution strategies
Resolving DMARC verification failures on subdomains requires a systematic approach, focusing on accurate SPF and DKIM configuration and alignment. It's not just about setting up a DMARC record, but ensuring that all legitimate email sending sources are authorized and that your chosen DMARC policy aligns with your sending practices.
By understanding the nuances of DMARC alignment (relaxed vs. strict), carefully reviewing your DNS records, and leveraging the insights from DMARC reports, you can effectively troubleshoot and fix these issues. This proactive management of your email authentication protocols ensures that your subdomain emails reach the inbox reliably, protecting your sender reputation and enhancing email security.
