Why does legitimate email fail DMARC even when doing everything right?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 Aug 2025
Updated 15 Aug 2025
8 min read
Many senders diligently set up SPF, DKIM, and DMARC, only to find that some of their legitimate emails still fail DMARC authentication. This can be a frustrating experience, especially when you feel you have "done everything right." The reality is that email delivery is a complex ecosystem, and several factors outside a sender's direct control can lead to DMARC failures, even for valid messages. Understanding these nuances is crucial for maintaining good email deliverability.
Email authentication protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) are designed to combat spam, phishing, and spoofing. While they are highly effective, their strict nature can sometimes penalize legitimate mail. The goal of DMARC is to tell receiving mail servers how to handle emails that purport to be from your domain but fail SPF or DKIM checks, providing strong protection against unauthorized use of your domain.
DMARC's core requirement is that either SPF or DKIM must align with the "From" domain (RFC5322.From) of the email. This means the domain used in the SPF check (the mailFrom or return-path domain) or the domain signed by DKIM must match the organizational domain in the "From" header. When this alignment breaks, even if SPF or DKIM technically pass, DMARC will fail.
One of the most common reasons for legitimate email to fail DMARC is email forwarding. When an email is forwarded, the path it takes often changes, leading to an alteration of the original message. This can break SPF, as the IP address of the forwarding server might not be authorized in the original sender's SPF record. Even if the forwarding server re-signs the email, if the DKIM signature is altered or a new one is added that doesn't align with the "From" domain, DMARC will still fail.
Similarly, intermediate mail transfer agents (MTAs) and email security gateways can introduce modifications that inadvertently cause DMARC failures. For instance, an MTA might add a "scanned for viruses" signature to the email body or alter headers, which can break the original DKIM signature. These modifications, though benign from a security scanning perspective, render the DKIM signature invalid, causing DMARC to fail.
Message modifications and transit issues
It is important to understand that neither an SPF failure nor a DKIM failure necessarily indicates that something malicious has happened to the message. SPF fails if the sending IP address isn't authorized in the domain's SPF record, which commonly occurs during email forwarding. DKIM fails if the message's content (or specific headers) is modified after the signature is applied, as the hash calculation will no longer match. These failures are often due to standard email processing.
Consider a scenario where an email passes through several servers before reaching its final destination. Each hop has the potential to modify headers or the body, even slightly. While many changes are harmless, DKIM's cryptographic nature means any alteration, no matter how small, will invalidate the signature. This is part of its design for integrity, but it can unfortunately catch legitimate emails in its net.
Another issue can arise with "on behalf of" sending, where an email might technically originate from one domain (e.g., a mailing list or a CRM) but appear to be "from" your domain in the display name. If the underlying authentication (SPF or DKIM) does not align with your domain, DMARC will flag it. While these emails might be legitimate in their intent, they violate DMARC's strict alignment requirements.
Misconfigurations and policy settings
Sometimes, the issue lies not with external factors, but with the DMARC policy itself or its initial deployment. An overly aggressive DMARC policy, such as p=reject or p=quarantine, set without thorough testing and monitoring of DMARC reports, can inadvertently block or quarantine legitimate emails that fail authentication. This is a common pitfall for organizations implementing DMARC for the first time.
Syntax errors in your DMARC record, or in your SPF and DKIM records, can also lead to widespread legitimate failures. Even a small typo can render an authentication record invalid, preventing receivers from properly verifying your emails. Regularly reviewing your DNS records for SPF, DKIM, and DMARC is essential to catch these types of errors.
Furthermore, issues can arise when using shared IP addresses, especially with large email service providers (ESPs) like Amazon SES. While the ESP might manage the SPF and DKIM authentication for its platform, a complex setup involving multiple regions or subtle internal routing within the ESP's infrastructure can lead to unexpected alignment failures in DMARC reports. This means legitimate mail sent from verified sources might still show DMARC failures due to how the ESP handles internal mail flow or applies its own default DKIM signatures.
Common DMARC misconfigurations
Aggressive policies: Setting p=reject or p=quarantine without first analyzing DMARC reports in monitoring mode (p=none).
Incomplete SPF records: Not including all legitimate sending sources in your SPF record, leading to hard fails for authorized emails.
Incorrect DKIM setup: Publishing an invalid DKIM public key or not rotating keys regularly.
Subdomain handling: Forgetting to set up DMARC for all active subdomains.
DNS and transient failures
Beyond explicit policy or content modification issues, the underlying infrastructure can contribute to DMARC failures for legitimate emails. DNS (Domain Name System) plays a critical role in email authentication, as both SPF and DKIM records are retrieved via DNS lookups. Transient DNS issues, such as temporary server outages, network latency, or even DNS rate-limiting by larger email platforms like Gmail, can prevent recipient mail servers from successfully retrieving your domain's authentication records.
If a receiving server cannot fetch your SPF record or DKIM public key due to a temporary DNS issue, the email might fail authentication, leading to a DMARC failure. While such occurrences are usually brief, they can impact a small percentage of your legitimate mail, especially if you send high volumes. This is an "expected and understood" failure mode that DMARC reports are designed to help you identify.
Large volume senders, or those using shared hosting environments where DNS is managed alongside other services, can be particularly susceptible to these transient DNS lookup issues. The sheer volume of authentication checks performed globally means that occasionally, a lookup will fail, not due to a misconfiguration on your part, but simply due to network flakiness.
For instance, some configurations with email security solutions like Proofpoint or Microsoft 365 can introduce complexities. If a filtering gateway sits in front of the primary mail server, it might alter the message or its path in a way that breaks authentication for incoming mail, or it might incorrectly handle internal handoffs, leading to SPF failures. While this is primarily an issue for inbound mail to an organization, it highlights how external systems can impact the perceived legitimacy of a sender's email.
Therefore, even when you adhere to best practices and configure everything correctly, a small percentage of legitimate emails may still fail DMARC. This isn't always a sign of malicious activity but can be an indicator of common challenges within the broader email ecosystem. Monitoring DMARC reports is key to discerning whether failures stem from genuine threats or these benign, yet impactful, operational quirks.
SPF failures (Return-path mismatch)
When an email is forwarded, the return-path address is often changed to the forwarding server. Since the forwarding server's IP is not in the original sender's SPF record, SPF authentication fails.
This is a common issue with mailing lists or email aliases.
DKIM failures (Message alteration)
DKIM relies on a cryptographic signature that verifies the integrity of the email headers and body. Even minor modifications, such as adding a footer or virus scan notification, can invalidate the DKIM signature.
Some mail servers or security appliances might rewrite email content in transit, breaking DKIM.
What is DMARC's purpose?
DMARC is designed to enable senders to indicate that their emails are protected by SPF and DKIM, and to tell receivers what to do if an email fails these checks. It also provides a feedback mechanism through DMARC reports, allowing senders to gain visibility into their email ecosystem.
The role of DMARC reports
DMARC aggregate reports (rua tag) provide a comprehensive overview of your email traffic, including authentication results for both passing and failing messages. These reports are invaluable for identifying legitimate email streams that might be unexpectedly failing DMARC. Forensic reports (ruf tag) provide more detailed information about individual failures, although they are less commonly implemented due to privacy concerns. Analyzing these reports is key to understanding and mitigating unexpected DMARC failures.
Continuously monitor your DMARC reports to identify legitimate email streams that are failing authentication.
Implement DMARC gradually, starting with a 'p=none' policy to gather data before enforcing stricter policies.
Work closely with third-party senders to ensure their SPF and DKIM configurations align with your domain's DMARC policy.
Ensure your DNS infrastructure is robust and can handle high query volumes to prevent transient lookup failures.
Common pitfalls
Deploying a strict DMARC policy (p=quarantine or p=reject) without first monitoring and understanding traffic patterns.
Ignoring DMARC reports, thus missing critical insights into legitimate failures and potential spoofing attempts.
Assuming all DMARC failures indicate malicious activity, rather than investigating potential benign causes.
Failing to account for email forwarding chains, which commonly break SPF and can lead to DMARC failures.
Expert tips
DMARC's core value lies in its feedback mechanism, allowing senders to identify and mitigate authentication issues.
A low rate of legitimate DMARC failures is expected due to the nature of email routing and modifications in transit.
For business mail streams, strict DMARC enforcement is crucial due to heightened concerns about phishing and malware.
Shared IP addresses, especially with large ESPs, can introduce complexities that lead to DMARC alignment issues.
Expert view
Expert from Email Geeks says DMARC is part and parcel of how DMARC works. Even if you do everything right, some of your legitimate mail will fail DMARC and be rejected.
2021-02-03 - Email Geeks
Marketer view
Marketer from Email Geeks says any type of programmatic message forwarding or modification risks breaking DMARC.
2021-02-04 - Email Geeks
Navigating the complexities of email authentication
Understanding why legitimate email fails DMARC requires a deep dive into the intricacies of email authentication and the dynamic nature of email delivery. It is rarely a simple "pass" or "fail" scenario. By recognizing that factors like forwarding, benign message alterations, and transient DNS issues can impact DMARC results, senders can approach troubleshooting with a more informed perspective.
The key is to leverage DMARC's reporting capabilities. These reports offer the visibility needed to distinguish between malicious activity and expected, yet problematic, legitimate traffic. Armed with this data, you can refine your DMARC policy, work with third-party senders, and adjust your sending practices to minimize disruptions to your legitimate email flow.
Ultimately, while DMARC provides robust protection against email fraud, it demands a proactive approach to monitoring and adjustment. Embracing its complexities rather than being surprised by them will lead to stronger domain reputation and improved email deliverability.
Amazon SES. While the ESP might manage the SPF and DKIM authentication for its platform, a complex setup involving multiple regions or subtle internal routing within the ESP's infrastructure can lead to unexpected alignment failures in DMARC reports. This means legitimate mail sent from verified sources might still show DMARC failures due to how the ESP handles internal mail flow or applies its own default DKIM signatures.
Gmail, can prevent recipient mail servers from successfully retrieving your domain's authentication records.
