Ever had an important email bounce back or land in spam, even though everything seemed fine? You’re not alone. Many businesses struggle with DMARC (Domain-based Message Authentication, Reporting, and Conformance) failures—even when their emails are completely legitimate.
So, why does this happen? And more importantly, how can you fix it? Let’s break it down in simple terms.
What is DMARC and Why Does It Matter?
Before diving into the issues, let’s do a quick refresher on DMARC. DMARC is an email authentication protocol that helps prevent email spoofing and phishing attacks. It works alongside SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) to verify that an email truly comes from the sender it claims to be from.
Without DMARC, attackers could easily impersonate your brand and send fraudulent emails to your customers or employees. That’s why implementing DMARC is essential for email security and brand reputation.
But what if your emails still fail DMARC—even when they’re legitimate?
Common Reasons Why Legitimate Emails Fail DMARC
Even if your organization follows best practices, there are several reasons why your emails might still fail DMARC authentication. Let’s go through them one by one.
1. Misconfigured SPF Records
SPF helps verify that the email is sent from an approved server. However, SPF records have a limit of 10 DNS lookups. If your record exceeds this limit (due to multiple third-party services like marketing platforms, CRMs, or ticketing systems), your SPF check can fail, leading to DMARC failures.
Solution:
- Use SPF flattening to reduce excessive DNS lookups.
- Regularly audit your SPF record and remove outdated or unnecessary entries.
2. DKIM Signature Issues
DKIM ensures that emails aren’t altered during transit. But if DKIM isn’t set up correctly, DMARC authentication can fail.
Common DKIM issues include:
- No DKIM record present in the DNS.
- Using multiple DKIM selectors without proper management.
- Expired or rotated DKIM keys that haven’t been updated in the DNS.
Solution:
- Always generate and publish a valid DKIM record.
- Keep track of DKIM key rotations to ensure new keys are properly configured.
3. Forwarding Emails Can Break Authentication
If an email is forwarded through another mail server, the SPF check often fails because the forwarded email is coming from an unapproved IP address. DKIM may or may not pass, depending on whether the email was modified during forwarding.
Solution:
- DMARC relies on either SPF or DKIM to pass. So, having both properly configured increases the chances of your email being authenticated, even after forwarding.
- Consider using ARC (Authenticated Received Chain) to help preserve authentication when emails are forwarded.
4. DMARC Policy Set Too Strict Too Soon
A common mistake businesses make is jumping straight to a strict DMARC policy (e.g., “reject”) without properly monitoring email traffic. This can cause legitimate emails to be blocked if they don’t pass SPF or DKIM.
Solution:
- Start with a “p=none” policy to monitor failures before enforcing stricter policies like quarantine or reject.
- Use DMARC reports to analyze authentication failures and make necessary adjustments.
5. Third-Party Email Services Aren’t Aligned with Your DMARC Setup
Many companies use third-party email services (e.g., Mailchimp, Salesforce, Zendesk, etc.). If these services aren’t correctly authorized in your SPF record or don’t sign emails with DKIM, they may fail DMARC authentication.
Solution:
- Ensure that all third-party services you use are properly configured in SPF, DKIM, and DMARC settings.
- If possible, set up custom DKIM signatures for third-party email services.
6. Subdomains and Multiple Domains Not Properly Configured
If you have multiple domains or subdomains sending emails, each one needs to have the correct SPF, DKIM, and DMARC records. A misconfigured subdomain can lead to DMARC failures.
Solution:
- Ensure all sending domains and subdomains have properly configured authentication records.
- Use a wildcard DKIM record if you manage multiple subdomains.
7. Email Modifications During Transit
Some email security solutions or gateways (like anti-spam filters or data loss prevention tools) modify email headers. If DKIM is enabled, any small modification to the email can break the DKIM signature, causing DMARC to fail.
Solution:
- Work with your IT team to check whether email security tools are modifying headers.
- If necessary, adjust DKIM settings to minimize the impact of modifications.
How to Monitor and Fix DMARC Failures
Now that you know the possible reasons for failures, here’s what you can do to stay on top of DMARC issues.
1. Use DMARC Reports
DMARC generates reports (RUA and RUF) that help you understand which emails are passing or failing authentication. These reports are essential for diagnosing and fixing issues.
2. Gradually Implement a Stronger DMARC Policy
Instead of going straight to “p=reject”, start with “p=none” to collect data. Then, gradually move to “quarantine” and finally “reject” once you are confident everything is configured correctly.
3. Regularly Audit Your Email Authentication Setup
SPF, DKIM, and DMARC records need ongoing maintenance. Set a schedule to review and update them regularly.
4. Work with Email Security Experts
If you’re unsure about your setup, working with an email security provider can help ensure your authentication settings are correctly configured.
Final Thoughts
DMARC failures can be frustrating, especially when they impact legitimate emails. But understanding the common reasons behind these failures—and how to fix them—can make a big difference.
By carefully setting up SPF, DKIM, and DMARC, monitoring reports, and continuously optimizing your email security settings, you can ensure your emails reach inboxes while keeping your brand protected from email fraud.
