Why does legitimate email fail DMARC even when doing everything right?
Summary
What email marketers say12Marketer opinions
Email marketer from Mailhardener.com highlights that transient DNS issues can cause legitimate email to fail DMARC. If a recipient's mail server cannot resolve the sender's SPF or DKIM records due to a temporary DNS outage, the authentication checks will fail, even if the email is otherwise valid.
Email marketer from MXToolbox responds that exceeding the SPF record lookup limit (10 DNS lookups) can cause SPF to fail. Even if your SPF record is technically correct, exceeding this limit will result in an SPF 'PermError,' which can lead to DMARC failing even for legitimate email.
Email marketer from Reddit shares that one very common reason is email forwarding. Many people have forwarding set up in their email accounts, and this almost always breaks SPF. Although DMARC can use DKIM, many setup guides recommend SPF or DKIM, not SPF and DKIM - and this is where people make mistakes.
Email marketer from Quora responds that one of the primary reasons for legitimate emails failing DMARC is simple configuration errors. Incorrect SPF records, DKIM signatures not properly set up, or DMARC policies misconfigured can all lead to legitimate email being rejected.
Email marketer from EmailGeeksCommunity highlights that legitimate emails can fail DMARC if the content is modified in transit. Even small changes, like an email server adding a footer or altering the email's encoding, can break the DKIM signature and cause DMARC to fail.
Email marketer from EmailSecurityExperts forum explains that using third-party email services without proper setup can lead to DMARC failures. If the third-party service is not correctly configured to use your domain's DKIM signature and SPF records, emails sent through them may fail authentication.
Email marketer from email deliverability consultant website shares that content filters, which are used to scan emails for spam or malicious content, can inadvertently modify emails, leading to DKIM signature invalidation and DMARC failures. This is common with overly aggressive or poorly configured filters.
Email marketer from StackOverflow explains that if you are using mailing lists, they might modify the email content, which will break the DKIM signature. Also, the mailing list server might not be authorized to send email on behalf of your domain (SPF failure).
Email marketer from Email Geeks shares findings that, for shared IPs on Amazon SES, DMARC failures often occur because emails are only signed with Amazon SES's automated DKIM signature (d=amazonses.com), failing alignment. This issue was specifically affecting the smtp-out.eu-west-1.amazonses.com region IP pools.
Email marketer from Email Geeks shares that failing DMARC doesn't necessarily mean spoofing. It could indicate misconfiguration, use of a service without alignment, or DNS lookup failures. It can be a clue, but isn't proof that an email is not legitimate.
Email marketer from sendlayers forum mentions that many times ESPs add tracking headers, tracking pixels, or even promotional banners to the bottom of emails which causes DKIM to break. Often this is not configurable in the setting and there is nothing a sender can do about it without changing ESP.
Email marketer from Reddit explains that some setups, such as having email aliases or multiple servers sending email, can cause legitimate email to fail DMARC due to the variety of ways mail is routed. These configurations are often difficult to account for in SPF and DKIM records.
What the experts say6Expert opinions
Expert from Email Geeks clarifies the roles of DKIM and DMARC. DKIM makes a positive assertion that mail was sent by a domain, and its failure is meaningless. DMARC makes negative assertions, and email failing DMARC doesn't automatically mean it's illegitimate. It means the sender wants email to be rejected if there's any doubt about its origin. He also explains that DKIM is not about the content of the message, it is about associating a responsible domain with an email message.
Expert from Spam Resource highlights issues with indirect mail flow. Forwarding and mailing lists can easily break SPF, since the forwarding server isn't authorized to send mail for the original domain. Even if DKIM is in place, modifications by the forwarder will invalidate the signature, causing DMARC to fail.
Expert from Email Geeks states that neither an SPF failure nor a DKIM failure indicates anything has been changed on the message and that SPF and DKIM checks couldn’t happen due to DNS issues. Asserting that the only reason SPF or DKIM will fail is because the message was modified is, quite simply, a falsehood.
Expert from Email Geeks explains that deploying DMARC doesn't automatically make previously legitimate email illegitimate. Legitimate email, sent by a legitimate sender to a legitimate recipient, can still fail DMARC due to trivial changes. Redefining "legitimate" to solely mean "passes DMARC" is a flawed argument that avoids addressing the genuine issues with DMARC.
Expert from Email Geeks explains that DMARC causes failures of legitimate messages. Using information, there is no reason to suspect that the messages were fake. Steve Atkins agrees that the IP address being the same is a critical detail.
Expert from Word to the Wise, Laura Belin, mentions that DMARC failures can stem from sender reputation issues. Even with perfect authentication, if your sending IP has a poor reputation, recipients might still reject your mail. She suggests monitoring your IP reputation and working to maintain a clean sending environment.
What the documentation says5Technical articles
Documentation from Valimail.com explains that common causes of DMARC failure include email forwarding, mailing list issues, and problems with third-party senders. Forwarding often breaks SPF, while mailing lists can alter messages, invalidating DKIM. Third-party senders might not be properly aligned with your domain.
Documentation from Google explains that for indirect email flows, such as forwarding lists, you can make use of ARC (Authenticated Received Chain) to validate mail transfer agents who are forwarding the email have not maliciously altered the original message.
Documentation from Microsoft explains that email forwarding can invalidate SPF. SPF works by checking if the sending server is authorized to send emails for the domain in the 'MAIL FROM' address. When an email is forwarded, the new sending server's IP address might not be listed in the SPF record of the original domain, causing SPF to fail.
Documentation from DMARC.org responds that DMARC is designed to protect against unauthorized use of your domain. If legitimate email is failing DMARC checks despite proper configuration, it suggests a problem in the email's authentication path, such as forwarding issues, or problems with the email service provider.
Documentation from SparkPost addresses the issue of multiple domains and shared infrastructure. If you send email from multiple domains or use shared IP addresses, DMARC failures can occur if your authentication is not properly aligned across all domains and sending sources.