Why might an email provider not honor a DMARC p=reject policy?

Key findings

• DMARC policy as a suggestion: The p= parameter in a DMARC record serves as a suggestion to the receiving email server, not a strict command. Receiving providers retain the ultimate discretion over how to handle messages that fail DMARC authentication, even with a p=reject policy in place. This flexibility allows them to prioritize factors like legitimate email delivery over strict policy enforcement.
• Internal policies vary: Each email provider has its own set of internal policies and algorithms for processing incoming mail. These policies often involve a complex interplay of authentication results (SPF, DKIM, DMARC), sender reputation, content analysis, and user feedback. A DMARC fail might contribute to a spam score but not necessarily trigger an outright rejection if other factors indicate legitimacy.
• Email forwarding complexities: Forwarded emails are a common reason for DMARC failures, as the forwarding process can break DKIM signatures or alter SPF alignment. Some providers may opt to deliver these emails to the spam folder rather than reject them outright to prevent loss of legitimate communication. Learn more about how DMARC policy overrides can occur.
• Gradual enforcement of p=reject: Some major providers, like Microsoft, have historically treated p=reject as p=quarantine or have only recently begun to honor p=reject on an opt-in basis. This cautious approach is often taken to avoid disrupting legitimate mail flow while gradually increasing enforcement of DMARC policies. For further reading, explore Microsoft's stance on DMARC policy enforcement.

Key considerations

• Monitoring DMARC reports: Regularly review your DMARC aggregate reports to understand how various receiving mail servers are interpreting your policy. This data is invaluable for identifying legitimate mail that might be failing authentication and taking corrective actions. It also helps you understand how to troubleshoot DMARC reject policies.
• Gradual policy implementation: It is always recommended to start with a p=none DMARC policy, then transition to p=quarantine, and finally to p=reject. This phased approach, detailed in how to safely implement p=reject, allows you to monitor impact and resolve issues before potentially blocking legitimate emails.
• Direct communication: If you observe unexpected DMARC behavior from a specific provider, consider reaching out to their postmaster team. Direct communication can often provide insights into their specific policies and any ongoing issues.
• Holistic deliverability factors: Remember that DMARC is just one component of email deliverability. Sender reputation, content quality, and engagement metrics also play significant roles in inbox placement. A strong DMARC policy might be mitigated by poor sender reputation, leading to mail being sent to spam instead of being rejected.

Key opinions

• Expectation of strict enforcement: Many marketers assume that a p=reject policy means instant blocking for non-compliant emails, and are surprised when these messages land in spam folders instead.
• Confusion over behavior: There's often a lack of clarity on why a specific provider, like ProtonMail mentioned in a community thread, would accept and spam a message rather than outright reject it, despite the sender's explicit reject policy.
• Impact on spoofing: Marketers rely on p=reject to combat email spoofing. When it's not fully honored, it raises concerns about the effectiveness of their security measures. Understanding how DMARC p=reject combats spoofing is key.
• Need for communication: Marketers frequently find themselves needing to directly contact postmaster teams to understand specific provider behaviors, especially when legitimate test emails with p=reject end up in spam.

Key considerations

• Thorough testing: Before implementing a p=reject policy, marketers should conduct extensive testing across various email clients and providers to gauge how their emails are handled, as behavior can vary significantly.
• Understanding DMARC reports: Leveraging DMARC aggregate reports is essential for marketers to see how different receivers are processing their mail and to identify any discrepancies in DMARC verification failures.
• Impact of forwarding: Marketers need to be aware that email forwarding can often break DMARC alignment, leading to legitimate emails failing authentication and potentially being sent to spam, even with a p=reject policy. This is a common scenario in many email flows.
• Monitoring inbox placement: It's important for marketers to monitor their inbox placement rates continuously. If emails are consistently landing in spam despite correct DMARC setup, it indicates that receiving servers are using other signals, like sender reputation, to make delivery decisions. Learn why your emails are going to spam.

Key opinions

• Receiver's discretion: Experts agree that receiving mail servers are ultimately free to handle email as they see fit, even if it means not strictly honoring a sender's p=reject policy. This sovereignty is fundamental to how the email ecosystem operates.
• Prioritizing legitimate mail: Many providers choose to err on the side of caution, routing DMARC-failing messages to spam rather than outright rejecting them. This prevents legitimate emails (especially forwarded ones) from being lost entirely.
• Historical practices: Some large mailbox providers, like Microsoft, have a history of treating p=reject as p=quarantine for years, indicating a deliberate policy choice rather than a technical oversight. This is why it's important to know how to handle Microsoft treating DMARC reject as quarantine.
• Complexity of mail flow: The email landscape is complex, with various intermediary services, forwarders, and security solutions that can impact DMARC validation. Experts acknowledge that these factors can lead to legitimate email failing DMARC without being truly malicious.

Key considerations

• Understanding DMARC purpose: Experts advise that DMARC is primarily a reporting and policy suggestion mechanism. Senders should use it to gain visibility into their sending practices and potential abuse, rather than assuming it will enforce an absolute rejection rule.
• Comprehensive authentication: Relying solely on DMARC p=reject for security is insufficient. A robust email security posture involves proper implementation of SPF and DKIM, in addition to DMARC, and ongoing monitoring. Learn more with a simple guide to DMARC, SPF, and DKIM.
• Postmaster engagement: When facing consistent issues with a specific provider, experts recommend engaging directly with their postmaster teams for clarification and to understand their unique handling of DMARC policies.
• Gradual policy transition: The consensus among experts is to transition DMARC policies gradually from p=none to p=quarantine and finally to p=reject. This allows time to address underlying authentication issues and ensures legitimate mail is not inadvertently blocked. Our guide on safely transitioning DMARC policy provides more detail.

Key findings

• DMARC specification: The DMARC specification defines p=reject as a policy where the receiver should refuse to accept the message during the SMTP transaction. This is the ideal scenario for senders seeking maximum protection against spoofing.
• Implicit discretion: While p=reject is a strong recommendation, underlying email protocols and the nature of spam filtering grant receiving mail servers autonomy. Documentation from various providers implies they may apply additional filtering layers or reputation checks that could override a pure DMARC reject decision.
• Forwarding challenges: Technical documentation often highlights that email forwarding can invalidate SPF and DKIM, leading to DMARC failures. Many providers choose to deliver such mail to spam to avoid blocking legitimate communications that simply passed through a forwarding service.
• Provider-specific implementations: Major email providers publish their specific DMARC enforcement policies, which can sometimes diverge from the most stringent interpretation of p=reject. Microsoft's past behavior of treating p=reject as p=quarantine is a notable example documented in their updates.