Summary
When sending internal emails through a third-party email service provider (ESP), it's a common and frustrating experience to have them flagged as impersonation attempts, even when SPF and DKIM authentication are seemingly correctly configured. This often occurs due to how internal mail flow and security filters within your organization (like Microsoft 365 or gateways such as Messagelabs) interpret incoming mail that originates externally but uses your internal domain in the 'From' address. While external recipients (like Gmail) might receive these emails without issue, internal systems can be much more sensitive, leading to false positives for impersonation.
Key findings
- Internal filter sensitivity: Your organization's internal email security filters are often more stringent and suspicious of emails that appear to originate from your domain but arrive via an external ESP, even if authenticated. This is a common setup to prevent actual internal spoofing.
- Authentication handoff: An internal mail gateway or layers (e.g., Messagelabs before Microsoft 365) might process emails in a way that interferes with or re-evaluates the authentication results, causing them to fail locally even if they passed upon initial external receipt. This can lead to a discrepancy between external and internal authentication reports. For more on this, read our guide on why O365 flags third-party ESP emails as spam.
- DMARC alignment: While SPF and DKIM might pass individually, DMARC requires alignment between the 'From' header domain and the domains used for SPF or DKIM authentication. If these domains do not align (e.g., your 'From' domain is yourdomain.com but the SPF domain is esp.com), DMARC can fail, triggering impersonation warnings, even with a p=none policy. Learn more about DMARC alignment failures.
- Sender reputation (external): For external receivers like Gmail, even with correct SPF, DKIM, and DMARC, a domain's reputation for newly authenticated sending identities needs to be built over time. Sporadic sending or recent changes in ESP can lead to emails landing in spam folders initially, despite passing technical checks. This is part of a complex algorithm that assesses trust. The WP Mail SMTP blog discusses common reasons why Gmail displays suspicious message warnings.
Key considerations
- Internal vs. external: Differentiate between impersonation warnings from your internal IT systems and deliverability issues with external providers. They often have different root causes and solutions.
- Header analysis: Thoroughly analyze email headers, particularly Authentication-Results, Return-Path, and DKIM-Signature, from both internal and external receipts. This helps pinpoint where authentication is breaking.
- DMARC policy: While a p=none DMARC policy will prevent rejections or quarantines, it still reports on alignment failures. Monitor your DMARC reports closely to identify if alignment issues are contributing to the impersonation flags.
- ESP configuration: Ensure your third-party ESP is configured for optimal DMARC alignment, often by signing with a DKIM record that uses your sending domain or by allowing you to customize the SPF domain. Confirm all IPs used by the ESP are included in your SPF record.
- Internal whitelist: Work with your IT department to whitelist your third-party ESP's sending IPs or domains within your internal mail filters. This can prevent unnecessary impersonation flags for legitimate internal communications.
What email marketers say
Email marketers often find themselves caught between the need to use third-party platforms for campaigns and the strict internal security protocols of their own organizations. The consensus among marketers facing internal impersonation flags is that while the immediate concern is for internal recipients, the broader worry is about how these flags might impact deliverability to external audiences. They often observe that external systems, like Gmail, may handle emails differently, sometimes placing them in spam due to evolving sender reputation rather than strict authentication failures.
Key opinions
- Internal vs. external flagging: Many marketers recognize that internal email security systems are designed to be highly cautious and might flag emails for impersonation simply because they originate from an external service, even if properly authenticated. They believe this specific issue often does not reflect broader deliverability problems with external mailboxes.
- Google's spam algorithms: There's a common sentiment that once SPF and DKIM are correctly set up, deliverability to providers like Gmail largely falls into the realm of proprietary spam algorithms. Marketers often feel that these algorithms operate on 'unknowable' factors beyond basic authentication, focusing heavily on sender reputation.
- Reputation building: Marketers understand that establishing a good sender reputation, especially with new or newly authenticated sending identities, takes time and consistent sending volume. Sporadic sending patterns or changes in ESPs can hinder this process, regardless of technical authentication passes.
- DMARC importance: Even with a passive DMARC policy (p=none), marketers acknowledge that DMARC's reporting capabilities are invaluable for monitoring authentication results and identifying potential alignment issues that could impact deliverability. See our article on simple DMARC examples.
Key considerations
- IT collaboration: Effective communication with the IT department is crucial to understand internal filtering rules and implement necessary whitelisting to ensure legitimate marketing emails aren't flagged as impersonation internally.
- Monitoring external deliverability: Even if internal flags are resolved, continuous monitoring of inbox placement for major providers like Gmail is essential. Tools like Google Postmaster Tools can offer insights into domain reputation and spam rates. Find out how to use Google Postmaster Tools V2.
- Consistent sending volume: To build positive sender reputation, aim for consistent email sending volumes rather than sporadic, large blasts. This helps receiving mail servers accurately assess your sending patterns.
- Understanding fraud detection: A marketer from Practical 365 suggests that legitimate emails failing fraud detection checks often indicate an incorrectly configured Sender Policy Framework (SPF) record, emphasizing the need for meticulous setup of all authentication protocols for third-party sending services.
Marketer view
An email marketer from Email Geeks shares their experience, noting that their IT department has implemented filters to identify impersonation, leading to marketing emails sent through Pardot being flagged internally. They mention that SPF and DKIM are verified within Pardot's tools, but IT remains concerned about deliverability due to this internal flagging.
Marketer view
A marketer from Email Geeks seeks clarification on whether their internal system's flagging of impersonation would also apply to other external email systems. They are trying to understand if the issue is isolated to their corporate network or indicative of broader deliverability problems.
What the experts say
Email deliverability experts highlight that internal impersonation flags, despite valid SPF and DKIM, frequently stem from the unique configurations of enterprise mail flows. They emphasize that while external authentication may be perfect, internal gateways (or layers) can re-evaluate or even break authentication, leading to false positives within the company's network. Furthermore, establishing domain reputation for new sending patterns or authenticated identities is a gradual process that influences how external providers like Google perceive incoming mail.
Key opinions
- Internal mail flow complexities: Experts agree that internal email security setups often process incoming mail from external sources (even legitimate ones using the organization's domain) as potential impersonation. This is especially true if there are multiple layers of mail processing, such as gateways before Microsoft 365, which can interfere with authentication results.
- Reputation establishment: A critical point is that when a domain starts actively authenticating with SPF, DKIM, and DMARC, the newly authenticated identifiers must build their reputation with receiving mail servers like Google. This process takes time and consistent sending volume to ensure optimal inbox placement. For more on this, see our article on how long it takes to recover domain reputation.
- DMARC alignment vs. SPF/DKIM pass: While SPF and DKIM might individually pass, experts stress that DMARC requires alignment of the 'From' header domain. If this alignment fails, it can trigger DMARC failures and associated impersonation warnings, even if the underlying SPF and DKIM checks are technically valid.
- Header analysis is key: To diagnose issues accurately, experts strongly recommend analyzing the full email headers, specifically the Authentication-Results and Received headers, to trace the email's path and identify where authentication failures occur. Our Email Deliverability Tester can assist with this.
Key considerations
- Identify authentication breaking points: Pinpoint whether the authentication failures are occurring at the external receiving end (less likely if SPF/DKIM are set up) or during internal handoffs within your organization's network.
- Whitelist internal traffic: For internal emails sent via third-party ESPs, establish explicit whitelisting rules within your organization's email security gateway (e.g., Microsoft 365, Messagelabs) to prevent legitimate emails from being flagged as impersonation.
- Consistent sending strategy: Maintain a consistent sending cadence and volume for your authenticated domains to foster a positive sender reputation with major ISPs. This is especially important after implementing or changing authentication protocols.
- DMARC monitoring: Implement DMARC with a p=none policy to gather insights from DMARC reports. This will provide visibility into authentication passes and failures, and crucial alignment issues, without impacting deliverability initially. An article on DuoCircle explains DMARC policies.
Expert view
An expert from Email Geeks advises that providing domain names and specific email headers, such as Authentication-Results, Return-Path, and DKIM-Signature, is crucial for diagnosing deliverability issues. This information allows for a detailed analysis of how email authentication is processed along the mail flow.
Expert view
An expert from Email Geeks points out that an internal gateway, like Messagelabs preceding an O365 layer, can interfere with authentication results. They explain that if O365 doesn't properly account for the gateway's processing, it may incorrectly report authentication failures, even if the external authentication was successful.
What the documentation says
Official documentation and industry research consistently underscore the critical role of email authentication protocols (SPF, DKIM, and DMARC) in combating impersonation and ensuring deliverability. However, they also implicitly or explicitly acknowledge that the interplay between these protocols, third-party sending, and internal mail security gateways can be complex. While external mail servers primarily rely on these standards for initial validation, internal systems often layer additional security checks that can lead to legitimate emails being flagged.
Key findings
- DMARC for spoofing control: DMARC is highlighted as the primary protocol for preventing outbound email spoofing and empowering domain owners to define how receiving mail servers should treat messages that fail authentication checks. It's crucial for strong anti-impersonation efforts.
- SPF and authorized senders: SPF is fundamental for email security by preventing unauthorized entities from impersonating a domain's email sender. It functions by allowing domain owners to list authorized sending IPs. Emails from third-party services can be flagged if their IPs are not properly included in the SPF record, as noted by AutoSPF's documentation on why legitimate emails fail SPF checks.
- DKIM for integrity: DKIM provides a cryptographic signature that verifies the sender's domain and ensures the email content has not been tampered with in transit. Its success is key for trust.
- Alignment is paramount: Even with SPF and DKIM passing, DMARC mandates alignment between the 'From' header domain and the SPF or DKIM authenticated domains. A lack of alignment can cause DMARC failures, leading to emails being treated as suspicious or spoofed, even by internal systems. For a detailed guide on this, see A simple guide to DMARC, SPF, and DKIM.
Key considerations
- Comprehensive DMARC implementation: Documentation often advises starting with a p=none DMARC policy to gather insights into third-party sender configurations and authentication patterns without immediately impacting deliverability. This data helps in identifying and resolving alignment issues.
- Vendor collaboration: Ensure all third-party ESPs are fully compliant with SPF, DKIM, and DMARC best practices, and that their sending IPs and DKIM signatures are correctly integrated into your domain's DNS records.
- Internal security policies: Review and adjust internal mail gateway settings to explicitly trust legitimate third-party senders, preventing false positive impersonation flags for internal recipients. This often involves whitelisting or creating specific transport rules.
- Consistent sender practices: Documentation emphasizes that consistent email sending practices are crucial for building and maintaining sender reputation, which is a key factor in deliverability for major email providers.
Technical article
Documentation from DuoCircle explains that if emails from a third-party vendor pass SPF and DKIM but fail DMARC, it often points to an alignment issue. The document emphasizes checking the SPF record for proper inclusion of third-party domains, and ensuring DKIM signatures align with the 'From' domain to prevent such failures.
Technical article
Documentation from AutoSPF states that SPF implementation is vital for preventing unauthorized individuals from impersonating a domain's email sender. It clarifies that SPF ensures only authorized servers listed in the SPF record can send mail on behalf of the domain, which is a cornerstone of email security and deliverability.
Related resources
15 resources
Related pages
Why does O365 mark emails from my domain sent via a third-party ESP as spam?
Why do my emails go to spam due to DMARC, SPF, and DKIM alignment failures?
Why are emails sent via an HR system connected to Gmail landing in spam despite authentication passing?
How can a phishing email pass SPF and DKIM authentication checks?
Why is my customer service email being flagged as spam by Google even with DKIM and SPF?
Why are some emails failing DMARC checks even with correct SPF and DKIM alignment, and how can I troubleshoot it?
How can I prevent brand and sender profile impersonation in emails and what actions can I take?
A simple guide to DMARC, SPF, and DKIM
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