Why am I seeing a 'Messages can be spoofed' warning in Outlook?

Key findings

• Internal security tools: The warning is often triggered by corporate email security tools like Proofpoint, which filter incoming mail for potential spam and phishing, even for internal company emails.
• Spoof intelligence: Microsoft's built-in Spoof Intelligence detects unverified senders, leading to the warning if authentication (SPF, DKIM, DMARC) is not fully aligned or is perceived as weak.
• DMARC policy impact: A missing DMARC record or a DMARC policy set to p=none can contribute to the Messages can be spoofed warning, as it provides less stringent guidance to receiving servers on how to handle unauthenticated mail.
• Recipient-specific settings: The warning may only appear for certain recipients if they have specific Outlook settings or corporate IT rules enabled that are not active for other users.
• Internal vs. external email: This warning is frequently observed with emails sent from within a company's own domain, especially marketing emails, as internal systems often apply stricter scrutiny.

Key considerations

• DMARC policy enforcement: To reduce these warnings, ensure your domain has a robust DMARC policy (e.g., p=quarantine or p=reject) in place, which instructs receiving mail servers on how to handle emails that fail DMARC authentication.
• Recipient IT consultation: The most direct way to resolve this for clients is to engage with their IT administrator, as they control internal email security settings and potentially override these warnings.
• Authentication alignment: Even if SPF and DKIM pass, DMARC alignment can still fail. Verify that your email authentication records (SPF, DKIM, and DMARC) are correctly configured and aligned, especially in the context of Outlook's sender requirements.
• Understand warning behavior: Recognize that these warnings are part of Microsoft's efforts to tackle spam and phishing, and may not always indicate a true spoofing attempt, but rather a lack of explicit authentication or an aggressive internal filter.

Key opinions

• Corporate security tools: Many marketers suspect corporate email security tools, such as Proofpoint, are the primary cause for these warnings, even for legitimate internal marketing emails.
• Internal IT department rules: The warning is often a result of specific rules set by the recipient company's IT department, which might differ from a standard Outlook setup.
• DMARC policy: Marketers recognize that a weak DMARC policy (or absence of one) can contribute to the Messages can be spoofed alert, emphasizing the need for proper authentication even for internal communications.
• Impact on email development: These warnings can significantly complicate email development and delivery efforts, especially when custom templates are involved.
• False positives: Marketers frequently encounter this as a false positive, particularly when troubleshooting Outlook deliverability issues for their campaigns.

Key considerations

• Check client's security stack: Before deep diving into technical authentication, inquire whether the client uses corporate email security products that might be causing the warning.
• Verify DMARC policy: Re-evaluate the DMARC record to ensure it's not set to p=none, even if SPF and DKIM appear to pass.
• Liaise with IT admins: If the issue persists, the next step should always be to consult the recipient's IT administration, as they control internal configurations that affect these warnings. This is particularly relevant when dealing with potential spoofing attempts within an organization.
• Test internal sending: Be aware that sending marketing emails (or any email) from your domain internally might trigger unique corporate anti-spoofing flags, which external recipients may not see.

Key opinions

• DMARC enforcement importance: Experts universally agree that a DMARC policy stronger than p=none is essential for preventing spoofing warnings and enhancing trust.
• Authentication alignment: The key is not just passing SPF and DKIM, but ensuring they align with the DMARC record's organizational domain. This alignment is what major mailbox providers, including Outlook, look for.
• Evolving anti-spoofing: Microsoft's Spoof Intelligence and other anti-spoofing technologies are constantly updated to combat sophisticated phishing and impersonation attempts, requiring senders to stay vigilant.
• Comprehensive monitoring: Regularly monitoring DMARC reports is crucial to identify legitimate email sources that might be getting flagged due to misconfigurations or perceived spoofing.
• Internal spoofing threat: Many organizations implement strict internal controls to prevent spoofing of their own domains, even by legitimate internal senders, leading to these warnings.

Key considerations

• Implement strong DMARC: Prioritize moving your DMARC policy from p=none to p=quarantine or p=reject to provide clearer instructions to receivers and reduce spoofing warnings. For guidance, see our article on safely transitioning DMARC policy.
• Consistent authentication: Ensure all legitimate sending sources (ESPs, transactional mailers) are correctly configured with SPF and DKIM, and that they achieve DMARC alignment.
• Domain reputation: Understand that domain and IP reputation also play a role. A strong reputation, built on consistent authentication and low complaint rates, can help mitigate these warnings.
• Benefits of DMARC: Beyond preventing spoofing, implementing DMARC provides valuable insights into email sending activity across your domain, making it a critical tool for deliverability. Explore the benefits of DMARC.

Key findings

• Spoof intelligence functionality: Microsoft's Spoof Intelligence is designed to identify and block spoofed messages by analyzing historical sender patterns and authentication results, among other indicators.
• Authentication standards: The warning often arises when SPF, DKIM, or DMARC authentication checks fail, or when DMARC's alignment requirements are not met.
• DMARC policy impact: A DMARC policy (especially p=quarantine or p=reject) is critical for instructing receiving servers on how to handle unauthenticated mail, thereby reducing spoofing perceptions.
• Types of spoofing: Documentation distinguishes between intra-org spoofing (within the same organization) and cross-domain spoofing, both of which can trigger these warnings depending on the mail flow.
• Visual indicators: Outlook adds specific tags (e.g., "via" tag, "unverified sender") to messages identified as potentially spoofed, providing visual cues to recipients.

Key considerations

• Refer to official documentation: Always consult Microsoft's official documentation for the most accurate and up-to-date information on their anti-spoofing mechanisms and recommended configurations, including specific DMARC tags.
• Implement DMARC correctly: Ensure that DMARC is configured not just for existence, but for proper alignment (strict or relaxed) and a policy that instructs receiving servers to quarantine or reject unauthenticated mail, aligning with best practices for fixing DMARC issues in Microsoft 365.
• Understand Exchange Online Protection (EOP): Recognize that EOP is a key component of Microsoft's email security, utilizing various features to protect against spoofing and phishing attacks.
• Internal overrides: Be aware that organizational IT departments can configure specific transport rules or trusted sender lists that may override default anti-spoofing behaviors for internal emails.