Why does DMARC authentication fail when SPF and DKIM pass, and how can it be fixed?

Even when SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) records appear to pass their individual authentication checks, DMARC (Domain-based Message Authentication, Reporting, and Conformance) can still fail. This commonly occurs due to DMARC's fundamental requirement for 'domain alignment.' DMARC necessitates that the organizational domain of the email's visible 'From' header aligns with the domain authenticated by SPF, typically the Return-Path or MailFrom domain, or with the domain authenticated by DKIM, specified in its 'd=' tag. If these domains do not match, either strictly or with a relaxed subdomain match, DMARC will not pass, regardless of whether SPF and DKIM records are technically valid for their respective domains. This issue is particularly prevalent when sending emails through third-party services that might default to using their own domains for SPF and DKIM authentication, thereby preventing alignment with your primary sending domain. The solution almost always involves configuring your email sending infrastructure to ensure this crucial domain alignment.

DMARC failures, even when SPF and DKIM authentication technically pass, stem almost universally from a lack of domain alignment. While SPF validates the 'Return-Path' domain and DKIM verifies the domain in its 'd=' tag, DMARC specifically mandates that at least one of these authenticated domains must align with the domain present in the email's 'From' header. This alignment can be either strict (exact match) or relaxed (subdomain match). A common scenario for this misalignment occurs when organizations use third-party email service providers that default to authenticating emails with their own internal domains. Although these authentications are valid for the third-party's domain, DMARC will fail for your primary sending domain because the 'From' header domain does not match. Resolving these failures typically requires configuring the sender to ensure the 'Return-Path' and DKIM 'd=' domains match or are subdomains of your 'From' domain.

While SPF and DKIM may individually authenticate an email, DMARC's more stringent requirement for domain alignment is frequently the cause of failure. This means the visible 'From' header domain must align with either the domain authenticated by SPF (the 'Return-Path' or 'MailFrom' domain) or the domain in the DKIM signature's 'd=' tag. If these domains do not match, even in a relaxed subdomain context, DMARC will not pass. This issue is particularly common when sending emails through third-party platforms that often use their own default domains for authentication, thereby creating a misalignment with the sender's true domain. The solution consistently involves adjusting your sending configuration to achieve this critical domain alignment.

DMARC's functionality as an email authentication protocol extends beyond merely validating SPF and DKIM records; it critically assesses their alignment with the sender's visible 'From' header domain. A common reason for DMARC failure, even when SPF and DKIM individually pass, is this specific requirement for domain alignment. For SPF, the 'Mail From' or 'Return-Path' domain must align with the 'From' header. Similarly, for DKIM, the domain specified in the 'd=' tag of the signature needs to align with the 'From' header domain. If either of these alignment conditions, strict or relaxed, is not met, DMARC will flag the email as a failure. This often occurs when organizations utilize third-party email services that, by default, authenticate emails using their own internal domains, creating a mismatch with the user's primary 'From' domain. Addressing this issue consistently involves configuring the sending setup to ensure the necessary domain alignment.