Why are internal emails flagged as impersonation when using a 3rd party provider even with SPF and DKIM verification?
Michael Ko
Co-founder & CEO, Suped
Published 30 Jul 2025
Updated 19 Aug 2025
9 min read
It's a frustrating scenario many businesses face: you're sending internal emails, perhaps marketing communications or transactional alerts, through a third-party email service provider (ESP) like Pardot, HubSpot, or a similar platform. You've diligently set up your Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) records, and the ESP's tools confirm they are verified. Yet, your own internal email filters, particularly those powered by systems like Microsoft 365, flag these legitimate emails as potential impersonation or spoofing attempts. This raises immediate concerns about deliverability and security.
When internal emails from a third-party provider are flagged as impersonation despite SPF and DKIM verification, the issue often stems from how your internal email gateway or security layers process incoming mail. While SPF and DKIM validate the sending server and message integrity, a crucial piece of the puzzle, DMARC, often determines how receiving mail servers should treat emails that pass or fail these checks, especially regarding the visible 'From' address.
I've seen this situation countless times, and it typically boils down to a misunderstanding or misconfiguration of DMARC alignment and how different mail systems handle email authentication results, particularly when email flows through multiple hops or gateways before reaching the final inbox. Let's delve into why this happens and what steps you can take to resolve it.
Understanding the email authentication trio: SPF, DKIM, and DMARC
To understand why internal emails might be flagged, it helps to briefly recap the roles of SPF, DKIM, and DMARC. SPF validates if the sending IP address is authorized by the domain owner. DKIM verifies that the email hasn't been tampered with in transit by cryptographically signing it. Both are foundational for email security, but they don't directly address the visible 'From' address that users see, which is often the target of impersonation attempts.
DMARC builds upon SPF and DKIM by linking their authentication results to the 'From' header domain. This alignment is critical. If your ESP sends email where the domain in the 'Return-Path' (for SPF) or the 'd= domain' in the DKIM signature doesn't align with the visible 'From' header domain, the DMARC check will fail, even if SPF or DKIM technically passed. This is a common reason for emails being flagged for impersonation.
Implementing a DMARC policy allows your domain to tell receiving servers what to do with emails that fail alignment: none (monitor), quarantine (send to spam), or reject (block entirely). This policy, combined with proper SPF and DKIM setup, is the strongest defense against domain spoofing and brand impersonation. If you're encountering impersonation flags, particularly for internal emails, reviewing your DMARC setup, including the alignment, is often the first step.
Authentication Protocol
What it Does
Impersonation Protection
SPF (Sender Policy Framework)
Authenticates the sending IP address against a list of authorized IPs in your DNS record.
Prevents unauthorized senders from using your domain's envelope-from address.
DKIM (DomainKeys Identified Mail)
Adds a cryptographic signature to the email header, verifying content integrity and sender identity.
Ensures the email hasn't been altered and confirms the sending domain's authenticity.
Aligns the 'From' header with SPF/DKIM authenticated domains and defines policies for handling failures.
Crucial for preventing direct domain spoofing where the 'From' address is faked.
The internal mail flow problem
A common culprit for internal emails being flagged is when your organization uses an inbound mail gateway, like Microsoft Defender for Office 365 or a third-party security solution (e.g., MessageLabs, Symantec Email Security.cloud), *in front* of your primary email server. These gateways perform their own authentication checks. When an email is sent from your third-party ESP and passes through this gateway, the gateway might alter or re-process the email in a way that breaks the original SPF or DKIM authentication when it's then handed off to your internal mail server. For example, the IP address the internal server sees might not be the ESP's original sending IP, or the DKIM signature could be invalidated if the message headers are modified.
The critical point here is the 'internal handoff'. Your external email authentication (SPF, DKIM, DMARC) might pass perfectly when checked by the inbound gateway, but then the gateway might re-send the email to your internal Office 365 or Exchange server from its own internal IP. If this internal IP isn't explicitly included in your SPF record, or if the DKIM signature is broken by the gateway's processing, your internal server will see it as a failed authentication attempt. This is often indicated by compauth=none reason=905 in Authentication-Results headers.
To fix this, you often need to configure your internal gateway and/or email server to trust the inbound connection from your third-party ESP or your gateway. This might involve adding the gateway's sending IPs to your SPF record, or creating mail flow rules to bypass certain checks for mail coming from trusted internal sources, ensuring the original authentication results are preserved or trusted downstream.
Bypassing internal authentication for trusted senders
When your internal security gateway or Microsoft 365 flags emails that already passed SPF and DKIM with your third-party provider, it's often due to the internal re-evaluation of authentication. This can create false positives for impersonation or spoofing. To prevent this, configure your internal system to trust your third-party sender or your inbound gateway.
IP whitelisting: Add the sending IP addresses of your third-party ESP (or your inbound gateway's IPs if it's re-sending mail internally) to your internal email server's trusted IP list or connection filter policy.
Mail flow rules: Create transport rules in your email system (e.g., Exchange Online) to bypass spam filtering or impersonation checks for emails originating from your trusted ESP's domains or IPs. This can be complex, so ensure proper testing.
Header analysis: Examine the full email headers for clues, especially the Authentication-Results and Received-SPF fields, to pinpoint where the authentication is failing internally. The Authentication-Results-Original header (if present) can show external pass results that are later overridden.
The nuances of DMARC alignment
Even if your ESP indicates SPF and DKIM are 'verified,' it doesn't always guarantee DMARC alignment. For SPF alignment, the 'Return-Path' domain (also known as MailFrom or Envelope-From) needs to match your 'From' header domain (or a subdomain thereof). For DKIM alignment, the 'd= domain' in the DKIM signature needs to match your 'From' header domain.
Many third-party providers use their own domains for the Return-Path or DKIM 'd= domain' by default, which can cause DMARC alignment to fail even if SPF and DKIM pass individually. This is especially true if you are on a shared IP address, as the ESP might use a tracking domain for the Return-Path. To achieve alignment, you may need to configure a custom Return-Path or custom DKIM signing domain provided by your ESP, ensuring it's a subdomain of your main sending domain (e.g., sends.yourdomain.com) and that the necessary CNAME records are correctly published in your DNS.
If your DMARC record has a p=none policy, emails that fail DMARC alignment will still be delivered, but they may be subject to additional scrutiny or flagged by recipient filters, especially internal ones that are more sensitive. Moving towards a p=quarantine or p=reject policy requires robust DMARC alignment for all legitimate sending sources.
Even when email authentication is perfectly configured, external mail systems like Google (Gmail) and Yahoo (and internal systems with advanced threat protection) rely heavily on sender reputation and content filters. If you've recently changed ESPs or started sending higher volumes, your sending domain (or the new ESP's IP) might not have established sufficient trust with recipients yet. This can lead to emails landing in spam or being flagged as suspicious, even if they technically pass authentication. Gmail, for instance, is known for its rigorous filtering.
Sender reputation is built over time through consistent positive sending behavior, low spam complaint rates, and high engagement. New sender IPs or domains, or domains that suddenly increase sending volume, can be viewed with suspicion. This is a separate layer of filtering from authentication. A blocklist (or blacklist) listing for the shared IP address could also lead to deliverability issues, even if your domain itself has a good reputation.
Even if your ESP says SPF and DKIM are verified, the ultimate arbiter of inbox placement is the recipient's mail server. Internal security filters, especially those configured for strict anti-spoofing measures, might interpret external sending as an impersonation risk, even if it's legitimate. This is often an overzealous internal policy or a lack of configuration to exempt your ESP.
To improve this, monitor your domain's reputation using tools like Google Postmaster Tools (for Gmail sending) and gradually ramp up your sending volume, if applicable, to build trust. For internal issues, collaborating with your IT team to adjust internal security configurations is crucial.
Authentication passes
External validation: Your ESP confirms SPF and DKIM are correctly set up and pass checks from external receivers.
DMARC policy active: You have a DMARC record, perhaps at p=none, allowing mail to be delivered even if alignment fails.
Headers look good: Initial Authentication-Results in headers show SPF and DKIM passing for the ESP's domain.
Internal deliverability challenges
Internal re-evaluation: Your internal email gateway or security solution re-checks authentication, potentially breaking the original pass results or flagging it due to an internal DMARC non-alignment.
Sender reputation: Low sending volume or a recent change in ESP means your domain's reputation with specific ISPs (like Gmail for internal employees) hasn't fully matured yet.
Strict internal policies: Your IT department's internal filters are designed to be highly cautious against any perceived impersonation, leading to false positives for legitimate external senders using your domain.
Views from the trenches
Ensure your third-party provider is configured for DMARC alignment, not just SPF and DKIM authentication. This often means using a custom sending domain (subdomain) for the Return-Path and d= domain that aligns with your From header.
Collaborate with your IT department to understand your internal email security gateway's settings. You might need to whitelist the third-party ESP's sending IP addresses or domains within your internal network's mail filters.
Use DMARC reporting tools to monitor your email authentication results across all receivers, both external and internal. This will provide visibility into where authentication might be breaking down.
Bringing it all together for better deliverability
Navigating email deliverability, especially when using third-party providers for internal communications, can be complex. While SPF and DKIM are essential, they are just part of the picture. The key often lies in DMARC alignment and how your internal email gateways process authenticated mail.
By understanding the nuances of how DMARC works with your 'From' header, configuring your third-party sender for proper alignment, and working closely with your IT team to adjust internal security configurations, you can significantly reduce the chances of your legitimate internal emails being flagged as impersonation. Remember, consistent monitoring and testing are crucial to maintaining optimal email deliverability and protecting your brand.
Microsoft 365, flag these legitimate emails as potential impersonation or spoofing attempts. This raises immediate concerns about deliverability and security.
Google (Gmail) and 
Yahoo (and internal systems with advanced threat protection) rely heavily on sender reputation and content filters. If you've recently changed ESPs or started sending higher volumes, your sending domain (or the new ESP's IP) might not have established sufficient trust with recipients yet. This can lead to emails landing in spam or being flagged as suspicious, even if they technically pass authentication. Gmail, for instance, is known for its rigorous filtering.
