Why am I seeing a 'Messages can be spoofed' warning in Outlook?
Matthew Whittaker
Co-founder & CTO, Suped
Published 30 Jul 2025
Updated 15 Aug 2025
7 min read
Outlook's 'Messages can be spoofed' warning is a security feature designed to protect users from malicious emails, primarily phishing and spam. While its intent is to alert recipients to potentially deceptive senders, it can sometimes appear on legitimate emails, causing confusion for both senders and recipients. This warning indicates that Outlook couldn't definitively verify the sender's identity, or that there's a discrepancy in how the email's 'From' address aligns with its actual sending source.
Email spoofing is the act of altering the sender's information in an email header to make it appear as if the email originated from a different source. Attackers use this technique to impersonate trusted entities, such as a known company, a colleague, or a financial institution, to deceive recipients.
The primary goal of email spoofing is to facilitate phishing attacks, where criminals trick recipients into revealing sensitive information, clicking malicious links, or downloading malware. Because the email appears to be from a legitimate source, recipients are more likely to trust it. For more information, you can read about phishing and suspicious behavior in Outlook.
While often associated with malicious intent, the 'Messages can be spoofed' warning can also appear for legitimate emails. This typically occurs when a sender's email authentication records are misconfigured or absent, leading email providers like Outlook to flag the message as potentially deceptive, even if it's not. This is a common issue that impacts email deliverability.
How Outlook detects spoofed messages
Microsoft 365, including Outlook, employs sophisticated anti-spoofing protection known as "Spoof Intelligence." This system analyzes multiple aspects of an incoming email to determine if the sender is legitimate or if the message is a potential spoof. It looks beyond just the visible 'From' address to understand the true origin.
Key to this detection are email authentication protocols: Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting & Conformance (DMARC). These standards allow receiving mail servers to verify that an email claiming to come from a specific domain is authorized by that domain's owner. If these checks fail or are not properly implemented for the sending domain, Outlook's Spoof Intelligence may flag the message. You can learn more about DMARC, SPF, and DKIM.
A common factor contributing to these warnings is a missing or relaxed DMARC policy, especially one set to 'p=none'. This policy tells receiving servers not to quarantine or reject emails that fail authentication. While useful for monitoring, it doesn't provide strong enforcement, which can lead Outlook's systems to exercise caution and display the warning. For details on Microsoft's anti-spoofing, consult their Defender for Office 365 documentation.
Method
Purpose
Impact on Spoofing Detection
SPF
Verifies sender IP address authorization.
Helps prevent unauthorized use of your domain in the Return-Path.
DKIM
Verifies message integrity and sender identity.
Ensures message content hasn't been tampered with in transit.
DMARC
Policy for handling emails that fail SPF or DKIM alignment.
Instructs recipient servers on how to handle unauthenticated emails.
Why legitimate emails trigger spoofing alerts
One common scenario for internal emails is when an organization uses a third-party email security solution, like Proofpoint, or has custom mail flow rules configured within their Microsoft Exchange environment. These tools can sometimes add their own warnings, even to legitimate internal messages, if they perceive any anomaly or if a specific rule is triggered. This is a common challenge, and we have a guide on how to resolve Proofpoint issues.
Another frequent cause is when you send emails through third-party services, such as a CRM, marketing automation platform, or transactional email provider. While these services send emails on your behalf, they might not always align perfectly with your domain's authentication records, especially DMARC. If the 'From' address in the email header doesn't align with the domain used for SPF or DKIM validation, Outlook can flag it. This often contributes to authenticated emails being marked as unverified.
Furthermore, if your domain's DMARC record is missing or set to a 'p=none' policy, it signals to receiving mail servers that you're only monitoring your email traffic and not enforcing strict authentication. While this is a good starting point for DMARC implementation, it doesn't give Outlook enough confidence to bypass its spoofing warnings entirely, especially for domains that are frequently targeted by spoofing attempts. This means you might be seeing DMARC verification failed errors.
Misconfiguration impact
Even minor misconfigurations in your email authentication setup can lead to significant deliverability issues and persistent 'spoofed' warnings. Regularly auditing your SPF, DKIM, and DMARC records is crucial to prevent these problems and ensure your legitimate emails reach the inbox without unnecessary flags.
Addressing the 'Messages can be spoofed' warning
The persistent 'Messages can be spoofed' warning, even for legitimate emails, can have several negative consequences. It erodes recipient trust, making them hesitant to open your messages or click on embedded links. This directly impacts your engagement metrics, leading to lower open rates and click-through rates. Ultimately, it can harm your sender reputation.
To address this, the first and most critical step is to ensure your email authentication, especially SPF, DKIM, and DMARC, is correctly implemented and configured for all domains and sending services. Verify that your DNS records are accurate and that all legitimate sending sources are properly authorized. This includes ensuring your emails are not going to spam.
Secondly, consider strengthening your DMARC policy. While 'p=none' is a safe starting point, moving to 'p=quarantine' or 'p=reject' instructs receiving servers to treat unauthenticated emails more strictly, significantly reducing the chances of your domain being spoofed and potentially eliminating these warnings for properly authenticated mail. It is crucial to transition your DMARC policy safely.
Potential issues
No DMARC or p=none: Little control over how receiving servers treat emails from your domain, even when legitimate.
Increased phishing risk: Easier for attackers to spoof your domain, as receiving servers lack clear instructions.
Warning messages: Frequent 'Messages can be spoofed' alerts, eroding recipient trust.
Benefits
Enhanced control: Greater protection for your brand and recipients against spoofing.
Reduced phishing: Less chance of your domain being abused by malicious actors.
Improved trust: Fewer warnings, leading to better inbox placement and recipient engagement.
Proactive measures and internal policies
For organizations, internal IT departments play a crucial role in resolving 'Messages can be spoofed' warnings, especially when they occur on internal emails. Corporate security tools or custom Exchange rules are often the underlying cause, and these require administrative access and expertise to properly review and adjust. Collaborating closely with your IT team ensures that internal policies do not inadvertently flag legitimate communications, which might happen when you get put on a blocklist or blacklist.
Views from the trenches
Best practices
Implement SPF, DKIM, and DMARC for all sending domains and services.
Monitor your DMARC reports regularly to identify authentication failures and legitimate sources.
Communicate proactively with your internal IT admins about any email security tools or custom rules that might affect deliverability.
Common pitfalls
Leaving DMARC policy at 'p=none' indefinitely, providing weak protection against spoofing.
Failing to configure third-party sending services for proper DMARC alignment, causing legitimate emails to be flagged.
Ignoring spoofing warnings, assuming they only affect external recipients and not internal communications.
Expert tips
Regularly audit your DNS records to ensure all email authentication settings are accurate and up-to-date.
Educate your users on how to identify and report suspicious emails, even if they have warning banners.
Utilize email deliverability testing tools to verify how your emails are received across different mailboxes.
Marketer view
Marketer from Email Geeks says clients with corporate email security tools like Proofpoint often experience internal warnings on marketing emails, even when sent from their own domain.
2020-12-03 - Email Geeks
Marketer view
Marketer from Email Geeks says applying additional rules for incoming email warnings, especially if DMARC is absent or set to 'p=none', can cause the 'Messages can be spoofed' alert.
2020-12-03 - Email Geeks
Ensuring your emails are trusted
The 'Messages can be spoofed' warning in Outlook is a signal that your email infrastructure may need attention. While it serves a vital role in protecting users from malicious spoofing and phishing attempts, it also highlights when legitimate emails lack proper authentication. By implementing and maintaining robust SPF, DKIM, and DMARC records, collaborating with your IT team, and carefully configuring third-party sending services, you can ensure your emails are both secure and reliably delivered, building greater trust with your recipients.
Outlook's 'Messages can be spoofed' warning is a security feature designed to protect users from malicious emails, primarily phishing and spam. While its intent is to alert recipients to potentially deceptive senders, it can sometimes appear on legitimate emails, causing confusion for both senders and recipients. This warning indicates that Outlook couldn't definitively verify the sender's identity, or that there's a discrepancy in how the email's 'From' address aligns with its actual sending source.
