How does ProofPoint affect email authentication for organizational Outlook domains?
Michael Ko
Co-founder & CEO, Suped
Published 14 Jun 2025
Updated 17 Aug 2025
10 min read
When managing email for an organization, especially one using Microsoft Outlook, dealing with a third-party email security gateway like Proofpoint can introduce complexities to email authentication. Many organizations rely on Proofpoint to filter out spam, phishing attempts, and malware before emails reach their employees' inboxes. While this security layer is crucial, it often acts as an intermediary, which can inadvertently affect how standard email authentication protocols like SPF, DKIM, and DMARC behave, leading to deliverability challenges for your legitimate emails.
I often see scenarios where email authentication passes for external recipients, but fails specifically for internal organizational Outlook domains. This is a common point of confusion, stemming from how Proofpoint intercepts and processes emails before they are handed off to the final destination, in this case, Outlook. Understanding this flow is key to resolving authentication issues and ensuring your emails are delivered as intended, both internally and externally.
This guide will walk you through how Proofpoint interacts with email authentication mechanisms and what steps you can take to maintain strong email deliverability and avoid legitimate emails being flagged or even blocked. We'll explore the technical intricacies and offer practical solutions to help you navigate this complex landscape, ensuring your organizational communications are secure and reliable.
Proofpoint operates as a Mail Exchange (MX) record intermediary. This means that when an email is sent to your organization's domain, it doesn't go directly to your Microsoft 365 Outlook servers. Instead, it's first routed through Proofpoint's infrastructure. This interception allows Proofpoint to scan, filter, and apply policies to the incoming mail before it reaches your internal network or Microsoft 365 environment. This security measure, while highly effective in preventing threats, fundamentally alters the email path, which in turn impacts how email authentication protocols perceive the message.
Because Proofpoint is the first point of contact for inbound emails, it essentially becomes the new last hop for the email before it's delivered to your organization. This setup can sometimes bypass your third-party mail filtering, a scenario where attackers might try to send malicious content directly to Office 365, circumventing the MX record. Ensuring your email gateway is secure is paramount to prevent such bypasses, as highlighted by Practical365.
Practical365 discusses ensuring third-party filtering gateways are secure. Consequently, when Microsoft 365 receives an email, it sees Proofpoint's IP address as the sending server, not the original sender's IP. This shift in the sending IP can trigger authentication failures, particularly for SPF, which checks if the sending IP is authorized by the sender's domain. Without proper configuration to account for this relay, Microsoft 365 might interpret legitimate emails as spoofed or unauthorized, leading to them being marked as spam or even rejected.
How Proofpoint impacts SPF, DKIM, and DMARC
Proofpoint's position as an email gateway directly influences how SPF, DKIM, and DMARC validate incoming messages. Here's a breakdown of the specific impacts:
SPF (Sender Policy Framework): When Proofpoint processes an incoming email, it acts as a relay. The SPF check performed by the final recipient (Microsoft 365) will see Proofpoint's IP address rather than the original sender's. If Proofpoint's IPs are not included in the sender's SPF record, the SPF validation will likely fail, leading to an SPF hard fail or soft fail. However, for internal delivery within your organizational Outlook domain, this SPF failure is often expected because Microsoft 365 is configured to trust Proofpoint's IP ranges through inbound connectors, which we will discuss next.
DKIM (DomainKeys Identified Mail): DKIM relies on the email's headers and body remaining unchanged after the signature is applied. Proofpoint, as a security gateway, often modifies emails for various reasons, such as adding headers for spam scoring, rewriting URLs for security, or archiving. Any modification to the email after the original DKIM signature is created will invalidate that signature, causing DKIM validation to fail. This is why you might see DKIM failures for emails processed by Proofpoint, even if the original sender correctly signed them. This is a common reason why DKIM might fail on Gmail or other receiving platforms.
DMARC (Domain-based Message Authentication, Reporting, & Conformance): DMARC relies on either SPF or DKIM passing, along with their respective alignment with the From: domain. Since Proofpoint can cause both SPF and DKIM to fail, DMARC authentication will also fail. For internal emails processed by Proofpoint and then sent to Outlook, DMARC failures are expected but typically managed by internal configurations. However, for external recipients, these failures would lead to messages being rejected, quarantined, or moved to spam, depending on the DMARC policy. You can find a simple guide to DMARC, SPF, and DKIM for more detailed information.
The authentication breakdown with Proofpoint in front
When Proofpoint sits in front of your organizational Outlook domain, the authentication chain changes. The initial SPF and DKIM checks might pass at Proofpoint's level, but when the email is relayed to Microsoft 365, the original authentication headers can be altered or stripped, and the sending IP seen by Outlook is Proofpoint's, not the original sender's. This often results in Outlook marking emails as 'Unverified Sender' even when they were originally authenticated, affecting internal visibility and trust.
SPF Failure: The SPF record checks the originating IP, which becomes Proofpoint's IP. Unless specifically configured, this results in an SPF failure from Outlook's perspective.
DKIM Invalidated: Proofpoint's modifications to email headers or body content cause the original DKIM signature to break, leading to DKIM validation failures.
Configuration challenges and solutions for Outlook domains
To ensure email authentication works seamlessly with Proofpoint and organizational Outlook domains, specific configurations are necessary. The primary solution revolves around proper setup within Microsoft 365 to account for Proofpoint as a trusted intermediary. This usually involves configuring inbound connectors and understanding Authenticated Received Chain (ARC).
Inbound connectors in Microsoft 365: This is the most critical step. You must configure inbound connectors in your Microsoft 365 (Exchange Online) environment to explicitly trust Proofpoint's sending IP addresses. This tells Microsoft 365 that any email coming from these IPs should not be subjected to the same strict SPF or DKIM checks as direct inbound mail. Instead, Microsoft 365 should rely on Proofpoint's security checks, effectively excluding these authentication failures from being flagged as spam.
Authenticated received chain (ARC): ARC is a protocol that allows email intermediaries like Proofpoint to re-sign emails after modification, preserving the original authentication results. When Proofpoint processes an email, it can add an ARC seal that includes the original authentication status. Microsoft 365 can then validate this ARC seal to determine the email's authenticity, even if Proofpoint's modifications broke the original DKIM signature or if the SPF check failed due to the relay. You should configure trusted ARC sealers in your Microsoft 365 Defender settings.
Implementing these configurations helps ensure that legitimate emails are not mistakenly identified as spoofed or unverified within your Outlook environment. It is crucial for organizations to maintain a robust email authentication setup, understanding that a third-party gateway alters the traditional email flow. For deeper insights into configuring these settings, it's often beneficial to consult Proofpoint's documentation or contact their support services, as they provide specific guidance on integrating their platform with Microsoft 365. You can also explore how to resolve Proofpoint identifying authenticated emails as spoofed.
Maintaining deliverability and reputation
Even with Proofpoint in place, maintaining strong email deliverability and a good sender reputation is crucial. While Proofpoint handles inbound filtering, your outbound email authentication still needs to be properly configured to ensure your emails reach recipients without being flagged as spam or blocked (or blacklisted). This involves careful management of your SPF, DKIM, and DMARC records, as well as proactive monitoring.
DMARC monitoring and reporting: Regularly review your DMARC reports. These reports provide invaluable insight into authentication failures, including those caused by Proofpoint's intermediation. Analyzing these reports helps you identify legitimate email streams that are failing authentication and adjust your configurations accordingly. You can use DMARC reports from Google and Yahoo to troubleshoot effectively.
Whitelisting Proofpoint IPs in Microsoft 365: As mentioned, ensuring Microsoft 365 recognizes Proofpoint as a trusted source for incoming mail is critical. This prevents internal authentication failures and ensures that emails that have already passed Proofpoint's scrutiny are not re-flagged by Microsoft's filters.
Ongoing monitoring and adjustments: Email environments are dynamic. Changes in Proofpoint's configuration, Microsoft 365 updates, or even new sending services can impact your authentication status. Regularly review your email logs and authentication results to proactively identify and address any emerging issues. Keeping an eye on your sender reputation and avoiding common pitfalls that lead to a blocklist (or blacklist) designation is essential for successful email campaigns.
Ensuring smooth email flow with Proofpoint
Check your MX records: Verify that your domain's MX records correctly point to Proofpoint's servers, ensuring all inbound mail flows through the gateway as intended.
Configure inbound connectors: Set up Microsoft 365 connectors to trust Proofpoint's IPs, preventing false authentication failures for internal emails.
Implement ARC: Enable ARC sealing in Proofpoint and configure Microsoft 365 to validate ARC, preserving original authentication results.
Views from the trenches
Best practices
Always configure inbound connectors in Microsoft 365 to explicitly trust Proofpoint's IP ranges.
Implement ARC (Authenticated Received Chain) on Proofpoint to preserve original authentication results.
Regularly monitor your DMARC reports for insights into authentication failures, especially those originating from Proofpoint.
Common pitfalls
Not whitelisting Proofpoint IPs in Microsoft 365, leading to legitimate internal emails being marked as unverified.
Overlooking DKIM breakage due to Proofpoint's email modifications without implementing ARC.
Assuming that authentication issues are only external, failing to check internal Outlook domain deliverability.
Expert tips
Ensure Proofpoint is configured for outbound relay for your domain in its interface for seamless sending.
Remember that blocking domains and IPs makes troubleshooting harder for external email deliverability support.
Be aware that an inbound connector on Microsoft 365 can be configured to exclude authentication failures from spam for trusted sources.
Marketer view
Marketer from Email Geeks says they have Proofpoint and all authentications are breaking, noting the sending IP is not the configured one. This indicates a relay issue.
2024-04-24 - Email Geeks
Expert view
Expert from Email Geeks says that if Proofpoint is in front of the email flow, authentication failures are expected for incoming emails.
2024-04-24 - Email Geeks
Ensuring seamless email delivery
Proofpoint serves as a crucial line of defense for organizational email, particularly for Outlook domains, but its role as an intermediary necessitates careful configuration to avoid unintended authentication issues. By understanding how Proofpoint interacts with SPF, DKIM, and DMARC, and by properly setting up inbound connectors and leveraging ARC within Microsoft 365, organizations can ensure that legitimate emails are consistently authenticated and delivered. Proactive monitoring and adjustments are vital for maintaining a robust email security posture and seamless communication flow.
Navigating the complexities of email authentication in environments with third-party gateways like Proofpoint can be challenging, but it is entirely manageable with the right approach. Prioritizing these technical configurations will safeguard your organization's internal and external email deliverability, preventing legitimate messages from falling victim to security false positives or ending up in the spam folder.
This holistic approach ensures that your organization benefits from Proofpoint's advanced threat protection while maintaining optimal email deliverability and authentication integrity. For more details on email deliverability, check out our guide on how SPF records and corporate email filters affect authentication.
Microsoft Outlook, dealing with a third-party email security gateway like Proofpoint can introduce complexities to email authentication. Many organizations rely on Proofpoint to filter out spam, phishing attempts, and malware before emails reach their employees' inboxes. While this security layer is crucial, it often acts as an intermediary, which can inadvertently affect how standard email authentication protocols like SPF, DKIM, and DMARC behave, leading to deliverability challenges for your legitimate emails.
