DMARC False Positives: Causes, Fixes, and Prevention Guide
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Last Updated: 6 min read
DMARC is an email authentication protocol that builds on SPF and DKIM. It helps domain owners specify how receiving mail servers should deal with emails that fail authentication. The key DMARC policies are none, quarantine, or reject. They let you control whether unauthenticated emails should be delivered, sent to spam, or get blocked entirely.
A DMARC false positive occurs when a legitimate email is incorrectly marked as a “fail.” This can be due to misconfigurations and gaps in SPF, DKIM, or DMARC settings. Common causes include misaligned domains, missing DKIM signatures, or overly strict policies applied without proper monitoring. Due to these gaps, business emails may be lost or redirected to spam. This can have a significant impact on your business operations, the effectiveness of communication, and your brand image.
Key Takeaways
- DMARC false positives are when legitimate emails fail DMARC checks due to misconfigurations or alignment issues.
- These failures can lead to email delivery and deliverability problems, reputational damage, reduced ROI, etc.
- Common causes of DMARC false positives are SPF or DKIM misalignment, email forwarding, expired DKIM keys, and unauthorized third-party senders.
- Signs of false positives may appear in DMARC failure reports, bounce logs, or through the unexpected rejection of internal or partner emails.
- You can reduce or eliminate false positives by monitoring and adjusting DMARC policies, authorizing third-party services, and handling email forwarding.
Why Do DMARC False Positives Happen?
DMARC false positives may arise from a large number of factors.
Poor Management of DNS Records
DNS records act as instructions that help email receivers validate SPF, DKIM, and DMARC authentication. They enable DMARC to find the right addresses to check the authenticity and legitimacy of email senders. Properly managing your SPF, DKIM, and DMARC DNS records ensures that receiving servers can correctly authenticate your emails. Without a DMARC record, email receivers will not apply DMARC policy, leaving your domain vulnerable to spoofing
SPF/DKIM Misalignment
DMARC requires the domain in the “From” header to match the domains used in SPF and DKIM authentication. If your alignment policies are too strict, this can cause legitimate emails to fail in case these domains differ. More specifically, when the SPF-authenticated domain (typically from the Return-Path) or the DKIM signing domain doesn’t align with the ‘From’ address, it can cause DMARC to fail.
Email Forwarding
Forwarded emails often pass through intermediate servers that are not listed in the sender’s SPF record. This may cause SPF to fail. Forwarding typically breaks SPF, but DKIM signatures usually survive unless the forwarder or mailing list modifies the message (such as adding footers or headers). The gaps in either SPF or DKIM may cause DMARC to fail.
Mailing Lists
Mailing lists sometimes involve modifying emails by adding footers or headers. They might even rewrite the “From” address altogether. These changes break DKIM signatures and cause SPF misalignment. This is because the mailing list’s bounce address differs from the original sender, so DMARC fails.
Expired or Rotated DKIM Keys
DKIM keys do not expire automatically, but they should be rotated regularly. Some DKIM signatures may include an x= tag that sets a signature expiration time, although it’s optional and not widely enforced. Careful management is needed during key rotation to ensure that public keys remain published until all emails signed with the old key have been delivered.
Use of Dynamic IP Address
It’s not at all surprising that DMARC performs best with stable IP addresses.. Dynamic IP addresses often shift and make it challenging to get to the intended destination. That is why a dynamic IP address is more likely to get blocked by email reputation services. What’s more, a dynamic IP address is likely to have inconsistent reverse DNS records. This, in turn, makes them more problematic and gives preference to stable IP addresses.
Unauthorized Third-Party Senders
Emails sent by third parties that are not included in SPF or DKIM records of the domain will most likely fail authentication. This may cause DMARC to fail and may potentially trigger false positives even if these senders are legitimate; if they are not properly authorized, then being legitimate isn’t enough.
How to Detect DMARC False Positives
Here are some ways you can detect DMARC false positives.
DMARC Aggregate Reports (RUA)
When you review your DMARC aggregate (RUA) reports, you can easily see and monitor pass and fail trends in your email communications. These XML-based reports are sent every day to the address that you have mentioned in your DMARC record. They include a summary of your authentication results for both SPF and DKIM, so that you can detect any unusual rise in failures. Such an unexpected rise may be a good indicator of false positives.
Forensic Reports (RUF)
Enabling DMARC forensic (RUF) reports will provide you with detailed, real-time diagnostics on all emails that fail DMARC checks. The reports are quite comprehensive and include message-level information. They may include sender data, subject, and authentication results. Because of the depth of data found in these emails, it becomes much easier to identify which legitimate emails are being sent to spam or rejected.
Email Bounce Logs
Checking your mail server’s bounce or rejection logs is another good method to spot false positives. If bounces get repeated, or if you experience frequent rejections of internal or partner emails, then your email ecosystem might be suffering from DMARC false positives.
Common Symptoms
Do your legitimate internal or partner emails disappear or get rejected for no reason? If you still experience delivery failures even when you have correctly configured SPF, DKIM, and DMARC records, then this is another sign of false positives.
How to Fix & Prevent DMARC False Positives
There are several effective methods for preventing and/or fixing false positives.
Ensure SPF/DKIM Alignment
For SPF, ensure the Return-Path (MAIL FROM) domain aligns with the visible From address. For DKIM, ensure the d= domain aligns with the visible From address. Alternatively, consider switching from strict alignment to relaxed if you need an easy fix to the false positives problem.
Authorize Third-Party Services
Another good method is to update your SPF records to include all legitimate third-party senders. You can do so either by using the include mechanism or listing their sending IPs. Make sure these vendors generate DKIM keys for your domain or subdomain and publish the public keys in your DNS.
Ensure Your DNS Records Are Well-Managed
To properly manage your DNS records, you can always use a DNS lookup tool to check the availability of a DMARC record. It should be a TXT record with your actual domain name.
It would also be useful to check the syntax of your DMARC record, so that all the directives are correctly formatted and are using the right punctuation (i.e., semicolons).
Handle Email Forwarding
We already learned that forwarding can break SPF and DKIM. In order to avoid this problem, you should implement ARC (Authenticated Received Chain). This can help preserve authentication results through forwarding hops. You can also configure forwarding services to add DKIM signatures, which can help avoid false positives, all while forwarding.
Leverage Proper Subdomain Management
A good DMARC deployment encompasses not just the domain but also subdomain handling. This means you should configure SPF, DKIM, and DMARC records for each of the subdomains and clearly specify the corresponding DMARC policies.
Monitor & Adjust Policies
Lastly, it’s important to know what DMARC policy you are using at any given stage of your DMARC implementation journey. For example, while a strict DMARC policy (e.g., p=reject) may be necessary in the later stages, it is recommended to start with a relaxed DMARC policy like p=none. Starting with the relaxed policy can help you collect data and identify false positives.
Once you have the necessary information, you can gradually move to quarantine and eventually reject. But only do so when you are confident that all legitimate sources are properly authenticated.
Best Practices to Reduce False Positives
Here is a quick checklist you can use to reduce or even eliminate false positives:
- Don’t directly move to p=reject when you have not yet done sufficient monitoring and testing. Patient, gradual DMARC enforcement can help you avoid the headaches associated with legitimate emails landing in spam.
- Regularly check and update your DNS records for SPF, DKIM, and DMARC. This will ensure all legitimate senders are covered.
- Always test your configuration with DMARC checkers before enforcing strict policies like p=reject. In addition to using online checkers, also ensure to carefully examine the reports to identify any errors or gaps.
- Avoid dynamic IP addresses (you already know why!).
- Collaborate with third-party vendors (such as CRMs and ESPs) to ensure their sending practices are DMARC-compliant and authorized for your domain.
Summing Up
False positives can be truly annoying, but in reality, they are easier to fix than you could ever imagine. Proper alignment, careful monitoring, and gradual enforcement of DMARC policies can help you detect and prevent them on time. Also, remember to audit your DMARC reports and adjust configurations for a hassle-free DMARC implementation and email delivery or deliverability.
Start using PowerDMARC’s free DMARC Analyzer today to monitor all your email authentication protocols under a single roof and get rid of false positives!
Domain & Email Security Expert at PowerDMARC
Ahona is the Marketing Manager at PowerDMARC, with 5+ years of experience in writing about cybersecurity topics, specializing in domain and email security. Ahona holds a post-graduation degree in Journalism and Communications, solidifying her career in the security sector since 2019.
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