What are the best practices for setting DMARC policy, particularly p=reject?

Key findings

• DMARC Goal: The primary objective of DMARC implementation is to reach a p=reject policy, ensuring that unauthenticated emails are blocked from recipients.
• Policy Actions: A p=reject policy directs mailbox providers to refuse messages that fail DMARC authentication, whereas p=quarantine sends them to the spam folder, and p=none only monitors.
• Security Benefits: A robust DMARC policy, such as p=reject, significantly protects an organization against email spoofing and phishing attacks.
• Phased Approach: Best practices recommend a gradual progression from p=none to p=quarantine, and finally to p=reject, often utilizing the pct tag to enforce policies on a percentage of mail.

Key considerations

• Accurate Mail Streams: Before moving to p=reject, ensure all legitimate email sources are properly authenticated with SPF and DKIM.
• DMARC Reports: Regularly reviewing DMARC aggregate and forensic reports is crucial to identify any legitimate emails that are failing authentication, as discussed in our guide on understanding and troubleshooting DMARC reports.
• Impact on Forwarding: Email forwarding can sometimes cause DMARC failures, especially with a p=reject policy, as the forwarded message may break SPF alignment.
• BIMI Requirements: To leverage BIMI (Brand Indicators for Message Identification), a DMARC policy of at least p=quarantine or p=reject is often required, as detailed in this article on DMARC implementation.
• Subdomain Policy: Understand whether your DMARC policy applies to your root domain, subdomains, or both. Our guide on DMARC policies for organizational domains and subdomains can provide more context.

Key opinions

• Confidence is Key: If a sender is highly confident in their email authentication (SPF and DKIM) for all outgoing mail, setting p=reject is seen as a viable and beneficial option.
• DMARC Policy Meanings: Marketers understand p=none for monitoring, p=quarantine for spam folder placement, and p=reject for outright blocking of unauthenticated mail.
• BIMI Necessity: For brands aiming to implement BIMI, a p=reject (or p=quarantine) policy is often a prerequisite, driving the push for stronger DMARC enforcement.
• Discovering Unknown Mail Streams: A significant challenge noted by marketers is identifying all legitimate email sending sources within an organization that may not yet be authenticated before moving to p=reject.

Key considerations

• Phased Transition: It is generally advisable to remain at p=none and thoroughly review DMARC reports for several months before progressing to stricter policies. See our guide on how to safely transition your DMARC policy.
• Impact on Redirects: Implementing p=reject can cause issues with email redirection, where messages from one mailbox are forwarded to another, potentially leading to DMARC authentication failures. This is especially relevant for discussion lists.
• Monitoring DMARC Reports: Even with high confidence in authentication, continuous monitoring of DMARC reports (via Google Postmaster Tools, for example) is crucial to catch any unexpected failures or spoofing attempts.
• Subdomain Scope: Verify if the DMARC policy is intended to apply to your primary domain, subdomains, or both, as this impacts its enforcement scope. This is important for ensuring a strong DMARC policy.

Key opinions

• Gradual Enforcement: Experts advise a slow and measured approach to DMARC policy enforcement, typically starting with p=none to gather data before moving to p=quarantine and then p=reject.
• Identify Mail Streams: A crucial step before p=reject is to use DMARC reports to identify all legitimate email sending services that use your domain and ensure they are properly authenticated.
• Mitigate Spoofing: Moving to p=reject significantly reduces the risk of third parties spoofing your domain for malicious purposes, protecting your brand reputation.
• Receiver Autonomy: Despite sender policies, the ultimate decision on how to handle DMARC failures rests with the receiving mail server, which may sometimes treat p=reject and p=quarantine similarly.

Key considerations

• Validation of All Sources: Ensure that all legitimate email sources, including third-party services (e.g., Google Workspace, recruiting apps, help desk software), are correctly configured with SPF and DKIM to align with your DMARC record before moving to enforcement.
• Review DMARC Reports: Actively review DMARC aggregate reports to identify any legitimate mail streams that are failing authentication, as these will be rejected once p=reject is in place. Our article on best practices for DMARC implementation provides more detail.
• Potential for Rejection: Be aware that some legitimate mail, particularly those affected by forwarding or complex routing, might still be rejected even with a correctly configured p=reject policy.
• Domain Scrutiny: Ensure the domains used in the From, Return-Path, and DKIM d= tags are correctly aligned, as misconfiguration can lead to blanket rejections.
• Cost/Benefit Analysis: While p=reject is the ideal, consider the effort required to reach this state versus the benefits, especially if there are many complex mail streams. More details can be found in this guide on DMARC best practices.

Key findings

• Policy Enforcement: A p=reject policy instructs email receivers to not accept messages that fail DMARC authentication, effectively blocking them.
• Protection against Spoofing: Enabling a DMARC policy, particularly p=reject, is crucial for protecting an organization against email spoofing and phishing, preventing malicious emails from reaching recipients.
• Phased Implementation: The recommended approach to DMARC implementation is a phased progression, starting with monitoring (p=none), then quarantining (p=quarantine), and finally rejecting (p=reject).
• PCT Tag Utilization: The pct tag is commonly used to advance policies, allowing senders to gradually increase the percentage of emails to which the DMARC policy applies, moving towards pct=100 with p=reject.

Key considerations

• DMARC Policy Significance: The DMARC p tag specifies how email service providers should handle incoming emails that fail DMARC authentication, offering reject, quarantine, or none as options.
• Avoid P=none Long-term: While useful for monitoring, a p=none policy should not be a permanent solution as it can pose security risks by not enforcing authentication. Our guide on simple DMARC examples explains initial setup.
• Troubleshooting Failures: If DMARC fails, common solutions include enabling or fixing SPF and DKIM authentication and adjusting your DMARC policy or authentication of your domain.
• Comprehensive Strategy: A full DMARC implementation strategy involves familiarizing yourself with policy implications, applying the pct tag, deliberately progressing policies, and ensuring ongoing DMARC maintenance. This forms part of the DMARC policy advancement.