What is DMARC p=reject?

DMARC p=reject is the strictest DMARC policy, instructing receiving mail servers to completely reject emails that fail DMARC authentication and alignment checks. This means the emails are not delivered to the recipient's inbox or spam folder.

How do third-party filters affect DMARC authentication?

Third-party email filters, like Mimecast or Proofpoint, can modify email headers or content, which might invalidate the original DKIM signature. However, well-configured filters often perform their own authentication or pass along authentication status (via ARC) to the final mail server.

Is it safe to use p=reject in B2B environments?

Yes, it is safe, but requires careful implementation. It's crucial to monitor DMARC reports thoroughly during the p=none and p=quarantine phases to ensure all legitimate mail sources are properly authenticated before moving to p=reject .

What should I do if my ESP doesn't support SPF for my domain?

If your ESP only uses DKIM for DMARC alignment, it's even more critical to monitor DMARC reports carefully. Ensure the DKIM signature remains intact through any third-party filters. If DKIM is consistently broken, you may need to reconsider the ESP or adjust configurations with the recipient's mail gateway if possible. DMARC needs at least one of SPF or DKIM to pass and align.

How can Suped help with DMARC p=reject implementation?

Suped provides AI-powered recommendations to guide you through DMARC setup and enforcement. Our platform offers real-time alerts, a unified view of DMARC, SPF, and DKIM monitoring, and specific features for MSPs, making it easier to identify and fix authentication issues before enforcing p=reject .

What are the considerations for DMARC p=reject in B2B environments with third-party filters? - DMARC - Email authentication - Knowledge base

The decision to implement a DMARC p=reject policy can feel daunting, especially for businesses operating in a B2B landscape. There's a common concern that third-party email filters, such as Mimecast or Proofpoint, might inadvertently break email authentication, leading to legitimate emails being rejected. This hesitation often revolves around ensuring critical communications reach their intended recipients without disruption.

Many initially consider two primary scenarios: if your Email Service Provider (ESP) authenticates with both SPF and DKIM, the SPF pass might ensure delivery even if DKIM records are altered by an intermediary. However, if an ESP relies solely on DKIM, a p=reject policy could indeed pose a risk, particularly when communicating with organizations that heavily utilize these filtering services.

While these are valid points, the nuances of email authentication in real-world B2B environments can be more complex. We need to look beyond general assumptions and delve into specific configurations and monitoring data to make informed decisions about DMARC enforcement. The goal is always to maximize security while minimizing the impact on legitimate email flow.

The basics of DMARC p=reject

At its core, DMARC p=reject instructs receiving mail servers to outright reject emails that fail DMARC authentication. This includes messages that don't align with SPF or DKIM. It's the strongest policy available, offering the highest level of protection against email spoofing and phishing attacks. However, this strength also means that misconfigurations or unexpected mail flow changes can lead to legitimate emails being rejected, which is a major concern in B2B settings where reliable communication is paramount.

Third-party email filters, such as

Mimecast or

Proofpoint, act as intermediaries, processing inbound emails before they reach the recipient's mailbox. During this process, they might modify email headers or the message body, which can inadvertently invalidate (or 'break') the DKIM signature. This doesn't mean these services are inherently bad, but rather that their integration requires careful consideration regarding email authentication protocols.

The common belief that SPF will always save the day if DKIM breaks is a simplification. While SPF alignment can certainly help, it's not a guaranteed bypass for DMARC failures, especially if the forwarding path introduces changes that also cause SPF to misalign. Furthermore, some legitimate email flows, particularly those from certain ESPs or services, might primarily rely on DKIM for DMARC alignment. Understanding how your entire email ecosystem functions, including all third-party services, is crucial for implementing DMARC safely.

Navigating third-party email filters

In B2B environments, it's very common for recipient organizations to use advanced email security gateways. These services inspect, rewrite, or even reroute emails, which can impact the integrity of authentication headers. For example, some filters might add a disclaimer to the email body, which breaks the original DKIM signature. The challenge isn't just about your sending practices, but also about how recipient systems process your mail.

However, most reputable third-party filters are designed with DMARC in mind. A properly configured filter will perform its own authentication checks upon receiving the email. If the email passes, the filter can then (in theory, via Authenticated Received Chain, or ARC) pass that authentication status to the final mail server, even if it modifies the email in a way that would otherwise break DKIM. This means the end recipient's mail server can trust the authentication performed by the intermediary, preventing legitimate emails from being rejected. The key lies in proper configuration on the recipient's side.

When considering the implications of using p=reject, it's important to remember that the issue isn't typically with your sending, but how the receiving infrastructure handles authentication after an intermediary. This is why gaining visibility into how your emails are processed downstream is so critical.

Before third-party filter

Direct mail flow: Emails sent directly from your ESP to the recipient's mail server.
Authentication status: SPF and DKIM signatures remain intact and align with DMARC policy.
DMARC check: DMARC policy is evaluated based on original authentication results.

After third-party filter impact on authentication

Intermediate processing: Emails pass through services like Mimecast or Proofpoint.
Potential DKIM breakage: Filters may modify headers or content, invalidating DKIM.
Re-authentication/ARC: Well-configured filters re-authenticate or pass original status.

Leveraging DMARC reports for insights

The single most valuable tool for addressing these concerns is comprehensive DMARC monitoring. Relying on assumptions about how third-party filters might impact your email authentication is a recipe for deliverability issues. DMARC aggregate reports provide granular data on all emails sent from your domain, indicating SPF and DKIM authentication statuses, DMARC alignment, and the policies applied by receiving mail servers.

These reports offer the 'hard data' needed to understand if legitimate emails are failing authentication and, if so, by whom and why. Before moving to a p=reject policy, you should spend ample time in a p=none or p=quarantine policy, carefully analyzing these reports to ensure all legitimate mail sources are properly authenticated and aligned. This phased approach allows you to identify and fix issues without impacting deliverability.

This is where a robust DMARC monitoring platform like Suped becomes indispensable. Suped provides clear, actionable insights from your DMARC reports, helping you quickly identify issues and understand how third-party filters might be affecting your authentication. Our platform offers AI-powered recommendations to guide you in fixing misconfigurations, alongside real-time alerts to notify you of any new authentication failures. This unified approach to DMARC, SPF, and DKIM monitoring, combined with blocklist and deliverability insights, ensures you have a complete picture of your email health.

Key Suped features for B2B DMARC implementation:

AI-Powered Recommendations: Get actionable advice to fix authentication issues and strengthen your policy.
Real-Time Alerts: Stay informed about new DMARC failures or policy changes impacting deliverability.
Unified Platform: Monitor DMARC, SPF, DKIM, and blocklist status from a single dashboard.
MSP and Multi-Tenancy Dashboard: Ideal for managing multiple client domains efficiently.

Safely transitioning to p=reject

The transition to DMARC p=reject should always be a deliberate, phased process. Starting with p=none allows you to gather data without affecting email delivery. Moving to p=quarantine provides another layer of caution, as emails failing DMARC will be sent to spam folders, giving you a chance to catch and resolve issues before outright rejection. Only once your DMARC reports show 100% legitimate email authentication, can you confidently switch to p=reject.

In B2B scenarios, this vigilance is even more critical. You must verify that all your legitimate sending sources, including marketing platforms, transactional email services, and CRM systems, are correctly configured for SPF and DKIM, and that their mail streams align with your DMARC policy. This also extends to understanding how your B2B partners' systems might handle your mail, though your primary control lies with your own sending infrastructure. With careful monitoring, transitioning to p=reject is achievable and highly beneficial for your domain's security.

Policy	Action on failure	Best for	Considerations
p=none	No action, email delivered normally.	Initial monitoring phase, gathering data.	No protection against spoofing. Essential first step.
p=quarantine	Email moved to spam/junk folder.	Testing phase, identifying legitimate failures.	Good for gradual enforcement. Potential legitimate mail in spam.
p=reject	Email completely rejected, not delivered.	Full protection against spoofing, after thorough testing.	Highest security. Requires careful monitoring to avoid false positives.

Views from the trenches

Best practices

Monitor your DMARC aggregate reports meticulously before enforcing p=reject to identify all legitimate sending sources.

Ensure all third-party email services are correctly configured with SPF and DKIM for your domain.

Implement DMARC in phases, starting with p=none, then p=quarantine, before moving to p=reject.

Common pitfalls

Skipping the monitoring phase and going straight to p=reject, leading to legitimate emails being blocked.

Not accounting for all third-party senders, resulting in DMARC failures for critical business communications.

Overlooking subtle DKIM breakage caused by intermediary mail filters without proper DMARC reporting.

Expert tips

Use a DMARC monitoring solution that provides clear, actionable insights to streamline the enforcement process.

Regularly review your DMARC aggregate reports to detect new sending sources or changes in authentication status.

Consider SPF flattening if you exceed the 10-lookup limit, which can cause SPF failures and DMARC alignment issues.

Marketer view

My company has successfully used p=reject for years, even in a B2B environment, and properly signed email is consistently delivered without issues.

2025-01-13 - Email Geeks

Marketer view

I agree that reviewing DMARC aggregate reports is absolutely essential to identify any DMARC failures before making policy changes.

2025-01-13 - Email Geeks

Making an informed DMARC decision

Implementing DMARC p=reject in a B2B environment, especially with the presence of third-party filters, is not without its considerations. However, the core message remains clear: with a thoughtful, data-driven approach, these challenges are surmountable.

The key is to leverage the power of DMARC reporting to gain complete visibility into your email ecosystem. By thoroughly understanding how your legitimate emails are being authenticated, even through intermediary filters, you can confidently progress to a p=reject policy. This not only enhances your domain's security but also protects your brand reputation by preventing unauthorized use of your email address.

Ultimately, the perceived risks of p=reject in B2B are often outweighed by the benefits of robust email security, provided you follow best practices and continuously monitor your email authentication. Tools like Suped are designed to simplify this process, making DMARC accessible and actionable for all organizations.