How to implement DMARC p=reject policy safely, especially when using ESPs like Mailchimp and GetResponse?

Key findings

• DMARC alignment: For DMARC to pass, an email must align with either SPF or DKIM, or both. If DKIM is aligned and passing consistently, it's generally sufficient to proceed with a p=reject policy, even if SPF alignment is not possible.
• SPF alignment with ESPs: Many ESPs, like Mailchimp, use their own domain in the return-path, which inherently breaks SPF alignment for your domain. This means that while your SPF record might be correctly configured to include the ESP's sending IPs, it won't align with your organizational domain for DMARC purposes.
• DKIM's robustness: DKIM is generally more robust against modifications in transit compared to SPF. It is less common for delivery paths to break DKIM signatures without also affecting SPF, though it can still happen (e.g., email forwarding).
• Transition strategy: A common and recommended approach is to start with a p=quarantine DMARC policy for a few weeks (2-3 weeks often suggested) to monitor DMARC reports for any unexpected legitimate failures before moving to p=reject.
• DMARC report coverage: DMARC reports (RUA records) are crucial for visibility into authentication results, but they only offer partial coverage, meaning some legitimate mail could still be discarded without appearing in reports if the receiving ISP does not send DMARC reports or handles DMARC differently.
• Benefits of p=reject: Beyond stopping email spoofing and protecting clients from phishing, a p=reject policy is often a prerequisite for implementing BIMI, which allows your brand logo to display next to your emails in supporting inboxes.

Key considerations

• Consistent data: Ensure your DMARC report data consistently shows successful DKIM alignment over several weeks or months before implementing p=reject. This provides a strong foundation for the stricter policy.
• Monitoring legitimate failures: Even with strong DKIM, unforeseen issues like DNS problems or in-transit modifications can cause legitimate emails to fail authentication. Regularly review DMARC compliance statistics to catch any legitimate messages failing DMARC.
• Forwarding implications: Emails forwarded by recipients (e.g., from Outlook.com to Gmail.com) can sometimes break DKIM signatures, potentially leading to DMARC failures. While consumer domains are generally good at not discarding such mail, it could still be spam-foldered.
• Gradual enforcement: For highly cautious implementers, the DMARC pct= tag can be used to enforce a DMARC policy on only a fraction of your mail stream, gradually increasing the percentage over time. This can be a step between p=quarantine and full p=reject.
• Internal communication: Inform your customer service representatives (CSRs) that a DMARC policy change has occurred. They should be prepared for potential I never got the email complaints, even if DMARC reports look perfect, as some legitimate mail might still be discarded due to nuances in DMARC enforcement by various mailbox providers.

Key opinions

• SPF alignment challenges: Many marketers using ESPs like Mailchimp find that SPF alignment (for DMARC) is often impossible due to the ESP's use of their own sending domains in the return-path, even if the SPF record itself is correctly configured.
• DKIM sufficiency: If DKIM is consistently aligned and passing across all sending sources, marketers feel confident in moving to a p=reject policy, understanding that DKIM can often carry the DMARC authentication alone.
• Phased implementation preference: A common practice among marketers is to start with a p=quarantine policy for a period (e.g., 2-3 weeks) to observe DMARC reports and ensure legitimate emails are not being impacted before graduating to p=reject.
• Fear of rejection: There is a natural apprehension about implementing p=reject due to the risk of legitimate emails being blocked, making a cautious approach with thorough monitoring highly desirable.
• BIMI as a driver: The desire to implement BIMI (Brand Indicators for Message Identification) serves as a significant motivation for many marketers to achieve a p=reject DMARC policy, recognizing it as a key requirement for brand visibility.

Key considerations

• ESP-specific configurations: Marketers need to understand how their chosen ESP handles SPF and DKIM authentication and DMARC alignment. Some ESPs may offer better SPF alignment options than others, sometimes requiring direct support interaction for advanced configurations.
• DMARC report analysis: Beyond Google Postmaster Tools, marketers should utilize comprehensive DMARC reporting tools to get a full picture of their email ecosystem, covering all mailbox providers, not just Google. This helps identify all legitimate sending sources.
• Impact of mail forwarding: Be aware that email forwarding can sometimes break DKIM signatures, which might lead to DMARC failures for legitimate emails. While consumer domains often have exceptions for known forwarders, there's still a risk of spam folding or rejection.
• Risk assessment: Consider the balance between the security benefits of p=reject and the potential (though rare) risk of legitimate emails being discarded. For some, the benefits of combating spoofing and enabling BIMI outweigh the minimal risk.
• DNS health: Ensure your DNS infrastructure is robust and reliable. DNS issues can inadvertently cause SPF or DKIM validation failures, leading to DMARC failures even with correct configurations. Marketers should refer to guides on setting up DMARC, DKIM, and SPF correctly.

Key opinions

• Robustness through both: While DMARC only requires one (SPF or DKIM) to align, experts suggest that having both SPF and DKIM align makes your email authentication more robust against messages being modified in transit. However, DKIM alignment alone is often sufficient in practice.
• Investigate SPF non-alignment: If SPF alignment is unexpectedly failing, experts advise investigating the root cause, especially when using ESPs that might alter the Mail From: domain. This understanding is key for long-term email health.
• DKIM rotation caution: When rotating DKIM keys, ensure a grace period of a few days before de-publishing old ones to prevent validation issues and potential DMARC failures.
• In-transit modification risks: Experts highlight that external factors, such as Security Email Gateways (SEGs) or user-configured forwarding rules (e.g., from Outlook to Gmail), can occasionally break DKIM signatures, risking legitimate mail rejection under a p=reject policy. However, these occurrences are becoming rarer.
• Phased rollout best practice: Starting with p=quarantine for 2-3 weeks, then transitioning to p=reject, is a widely endorsed cautious approach, assuming DMARC reports show minimal failures.

Key considerations

• DMARC compliance monitoring: Focus on the DMARC compliance status in your reports. If legitimate messages are not passing, they need to be configured correctly before moving to a stricter policy. This is critical for preventing deliverability issues and being added to a domain blacklist.
• Beyond Google Postmaster Tools: Relying solely on Google Postmaster Tools provides only a partial view of DMARC compliance. Experts recommend analyzing DMARC reports from a variety of recipient mailbox providers to get comprehensive coverage.
• Balancing risks and benefits: Organizations must weigh the risks of legitimate mail being silently discarded against the benefits of DMARC p=reject, such as enhanced anti-spoofing and the ability to implement BIMI.
• Stakeholder awareness: Those making the decision to move to p=reject must understand the inherent limitations of DMARC reports (partial coverage) and prepare customer support teams for potential complaints about missing emails.
• DNS health: Occasional DNS failures can lead to DMARC non-alignment. Continuous monitoring of your DNS infrastructure is important to prevent disruptions to email authentication.

Key findings

• DMARC authentication requirement: DMARC requires either SPF or DKIM to pass and align with the domain in the From: header of the email.
• SPF alignment: SPF alignment means the domain in the Return-Path (Mail From) header matches the organizational domain of the From: header. Many ESPs do not facilitate this direct alignment.
• DKIM alignment: DKIM alignment means the domain used to sign the email (the 'd=' tag in the DKIM signature) matches the organizational domain of the From: header. This is often the primary alignment method for emails sent via ESPs.
• Policy options: DMARC offers three policies: p=none (monitoring), p=quarantine (send to spam/junk), and p=reject (discard). Each policy has increasing levels of enforcement.
• Importance of reporting: DMARC aggregate reports (rua tag) and forensic reports (ruf tag) are critical for monitoring email authentication results and identifying unauthorized sending sources.

Key considerations

• Staged deployment: The recommended practice is a phased deployment, starting with p=none to gather data and identify all legitimate sending sources, then moving to p=quarantine, and finally p=reject once confidence is high that all legitimate mail will pass DMARC.
• Percentage tag (pct=): For a gradual rollout of enforcement policies, the pct= tag can be used to apply the DMARC policy to only a percentage of failing messages, allowing for a controlled transition to full enforcement. This can be especially useful for identifying additional legitimate sending sources as part of your DMARC enforcement strategy.
• Domain ownership and control: To correctly configure DMARC, organizations must have control over their domain's DNS records to publish the DMARC TXT record.
• Regular review of reports: Even after reaching p=reject, ongoing monitoring of DMARC reports is advised to catch any new sending sources or unforeseen authentication issues, ensuring continued legitimate email delivery and protection from blocklists.