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How to use DMARC p=reject to combat email spoofing and its potential impact on legitimate mail?

Summary

Implementing DMARC with a p=reject policy is the most stringent way to combat email spoofing and phishing attacks that abuse your domain. When a receiving server encounters an email claiming to be from your domain, but it fails DMARC authentication, a p=reject policy instructs that server to reject the email outright. This prevents the spoofed email from reaching the recipient's inbox, thereby protecting your brand's reputation and your recipients from malicious content.

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What email marketers say

Email marketers often approach the idea of a p=reject DMARC policy with a mix of enthusiasm for security and trepidation about potential deliverability issues. Their primary goal is to ensure legitimate marketing and transactional emails reach the inbox reliably. While they recognize the benefit of stopping brand spoofing and protecting their sender reputation, the fear of accidentally blocking their own emails due to misconfigurations or third-party sending nuances is a significant concern. Marketers typically rely on DMARC reports to gain visibility into their email ecosystem before considering stricter policies, often starting with p=none to monitor and refine their authentication.

Marketer view

Marketer from Email Geeks notes a sudden increase in spoofing. They explain that they hadn't previously observed emails spoofing their domain, but recently received numerous complaints about emails they didn't send, which were using their sending domain. This situation prompted their immediate inquiry into DMARC's effectiveness.

20 Oct 2019 - Email Geeks

Marketer view

Marketer from Reddit suggests switching to p=reject yielded immediate positive results. They note a noticeable reduction in phishing attempts after implementation, affirming its value for brand protection. However, they emphasize that the transition period necessitated diligent monitoring of DMARC reports to prevent any unintended loss of legitimate email traffic.

15 Sep 2024 - Reddit

What the experts say

Email deliverability experts consistently advise extreme caution when considering a p=reject DMARC policy. Their collective wisdom emphasizes that while p=reject is the most effective defense against domain spoofing, its implementation is fraught with potential pitfalls if not executed meticulously. Experts stress that a deep understanding of your email ecosystem, robust authentication for all sending sources, and continuous monitoring of DMARC reports are non-negotiable prerequisites. They frequently point out the complexities introduced by mail forwarders and mailing lists, which can inadvertently cause legitimate mail to be blocked under a strict DMARC policy.

Expert view

Expert from Email Geeks advises caution against immediate p=reject adoption. They strongly recommend against changing directly to p=reject without thorough preparation, implying the significant risks associated with such a hurried implementation.

20 Oct 2019 - Email Geeks

Expert view

Expert from Spamresource.com states that many organizations adopt p=reject too quickly, often without a comprehensive understanding of their email ecosystem. This premature move can result in a substantial loss of legitimate email. They emphasize the critical need for meticulous analysis of DMARC aggregate reports prior to enforcement.

01 Nov 2024 - Spamresource.com

What the documentation says

Technical documentation on DMARC policies consistently defines p=reject as the most aggressive and effective enforcement option for DMARC. It explicitly instructs receiving mail servers to decline (reject) any email that purports to be from a domain, but fails DMARC authentication. Documentation emphasizes that this policy offers the highest level of protection against domain spoofing and email-based fraud. However, it also critically highlights the prerequisite of having all legitimate email streams correctly authenticated with SPF and DKIM and achieving DMARC alignment. Without this foundational setup, legitimate mail can be inadvertently blocked, making thorough preparation and ongoing monitoring indispensable for successful implementation.

Technical article

Documentation from RFC 7489 specifies that the DMARC p=reject policy instructs receiving mail servers to discard messages that fail DMARC authentication checks for the organizational domain. This policy provides the highest level of protection against email fraud and spoofing attempts, ensuring that unauthorized emails are not delivered to recipients.

01 Mar 2015 - RFC 7489

Technical article

Documentation from M3AAWG Best Practices recommends a prerequisite for deploying p=reject. They state that organizations should possess a thorough understanding of all legitimate email sending sources linked to their domain, including third-party senders and internal applications. Comprehensive DMARC reporting analysis is identified as essential for this crucial preparatory step.

15 Sep 2023 - M3AAWG Best Practices

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