Summary
Legitimate emails can fail DMARC despite proper configuration due to a combination of factors related to email handling and authentication intricacies. Key issues include email forwarding and mailing lists, which often disrupt SPF and DKIM. Transient DNS problems, configuration errors, and the use of third-party senders without proper alignment can also cause failures. Content modifications during transit, exceeding SPF lookup limits, and certain email setups like aliases or multiple servers introduce further complications. Furthermore, automated DKIM signatures on shared IPs, like with Amazon SES, can lead to alignment issues, and sender reputation can impact deliverability. It's important to recognize DMARC's design and the potential for legitimate mail to fail authentication for reasons unrelated to malicious activity.
Key findings
- Indirect Mail Flow: Forwarding and mailing lists frequently break SPF and can invalidate DKIM.
- DNS Issues: Transient DNS outages prevent SPF/DKIM record resolution.
- Configuration Errors: Incorrect SPF/DKIM setups or DMARC policy lead to failures.
- Third-Party Services: Misaligned third-party senders cause authentication failures.
- Content Modification: Changes in transit (filters, headers) invalidate DKIM.
- SPF Limits: Exceeding SPF record lookups results in SPF failures.
- Complex Setups: Aliases/multiple servers complicate routing and cause failures.
- Shared IPs: Shared IPs with automated DKIM signatures on shared IPs lead to authentication issues
- DMARC Definition: DMARC makes negative ascertions and can reject messages when anything is not perfect.
- Sender Reputation: Poor sender reputation can lead to email rejection.
Key considerations
- Optimize Configuration: Regularly audit and correct SPF/DKIM/DMARC settings.
- Manage Forwarding: Discourage forwarding or use authentication like ARC.
- Authenticate Mailing Lists: Configure mailing lists for authentication or use ARC.
- Monitor DNS: Implement DNS monitoring and promptly resolve resolution issues.
- Align Third Parties: Ensure that all third-party senders align properly with your domain.
- Review Content Filters: Adjust filters to minimize unintended content changes.
- Reduce SPF Lookups: Optimize SPF records to stay within the lookup limit.
- Monitor Sender Reputation: Monitor sender reputation.
- Follow Setup Guides: Follow email security configuration guides to set up properly.
- In Transit Changes: Identify areas that could be making changes in transit
What email marketers say
12 marketer opinions
Legitimate emails can fail DMARC despite proper setup due to various reasons, including transient DNS issues, email forwarding (breaking SPF), mailing list modifications (breaking DKIM), misconfigurations, or use of third-party services without proper alignment. Even minor changes in email content during transit, such as those caused by content filters or ESPs adding tracking headers, can invalidate DKIM signatures. Exceeding SPF record lookup limits or using email aliases/multiple servers can also cause DMARC failures. Furthermore, shared IPs with automated DKIM signatures, like those from Amazon SES, can lead to alignment problems, and sender reputation can play a role.
Key opinions
- Transient DNS: Temporary DNS outages can prevent recipient servers from resolving SPF/DKIM records, causing authentication failures.
- Email Forwarding: Forwarding often breaks SPF, as the forwarding server isn't authorized to send mail for the original domain.
- Mailing List Modifications: Mailing lists can alter email content, invalidating DKIM signatures.
- Misconfigurations: Incorrect SPF/DKIM setups or DMARC policy misconfigurations can cause legitimate emails to be rejected.
- Third-Party Services: Using third-party services without proper DKIM/SPF alignment leads to authentication failures.
- Content Modification: Changes during transit (e.g., by content filters or ESPs) can invalidate DKIM signatures.
- SPF Lookup Limits: Exceeding the SPF record lookup limit causes SPF failures and impacts DMARC.
- Email Aliases/Servers: Complex setups with aliases or multiple servers can cause routing issues and DMARC failures.
- Shared IPs (Amazon SES): Shared IPs may cause DMARC failures due to incorrect DKIM signing and automated DKIM signatures failing alignment.
Key considerations
- Monitor DNS: Implement robust DNS monitoring to quickly identify and resolve any DNS resolution issues.
- Minimize Forwarding: Discourage or educate users on the impact of email forwarding on DMARC compliance.
- Authenticate Mailing Lists: Configure mailing lists to properly handle authentication or consider using ARC to preserve authentication results.
- Regular Audits: Perform regular audits of SPF, DKIM, and DMARC configurations to identify and correct errors.
- Proper ESP Configuration: Ensure any third-party email service is correctly configured with proper DKIM signatures and SPF records.
- Review Content Filters: Assess and adjust content filter configurations to minimize unintended content modifications.
- Optimize SPF Records: Optimize SPF records to stay within the lookup limit.
- Simplify Email Routing: Reduce complexity in email routing configurations to avoid SPF/DKIM failures.
- Investigate ESP: Investigate if the ESP is breaking DKIM with content inject, and seek alternative ESP
Marketer view
Email marketer from Mailhardener.com highlights that transient DNS issues can cause legitimate email to fail DMARC. If a recipient's mail server cannot resolve the sender's SPF or DKIM records due to a temporary DNS outage, the authentication checks will fail, even if the email is otherwise valid.
26 Oct 2021 - Mailhardener.com
Marketer view
Email marketer from MXToolbox responds that exceeding the SPF record lookup limit (10 DNS lookups) can cause SPF to fail. Even if your SPF record is technically correct, exceeding this limit will result in an SPF 'PermError,' which can lead to DMARC failing even for legitimate email.
31 Jan 2025 - MXToolbox
What the experts say
6 expert opinions
Legitimate emails can fail DMARC even when configured correctly due to several factors. DMARC's design emphasizes negative assertions, meaning that if any doubt exists about the email's origin, it might be rejected, even if sent by a legitimate sender. Indirect mail flow, such as forwarding or mailing lists, often breaks SPF and invalidates DKIM signatures. Sender reputation also plays a crucial role; a poor reputation can lead to rejection despite proper authentication. SPF and DKIM failures do not always indicate malicious activity but may stem from transient DNS issues or normal message modifications during transit.
Key opinions
- DMARC's Negative Assertions: DMARC can reject legitimate email if there's any doubt about its origin, even with correct configuration.
- Indirect Mail Flow Issues: Forwarding and mailing lists frequently break SPF and invalidate DKIM, causing DMARC failures.
- Sender Reputation Impact: Poor sender reputation can lead to email rejection despite proper authentication.
- SPF/DKIM Failure Meaning: SPF and DKIM failures do not necessarily indicate malicious activity but can be due to DNS issues or message alterations.
Key considerations
- Acknowledge DMARC Imperfections: Recognize that DMARC isn't perfect and can sometimes flag legitimate email.
- Manage Indirect Mail Flows: Implement strategies to handle forwarding and mailing lists properly, such as ARC.
- Monitor Reputation: Actively monitor and maintain sender reputation to ensure deliverability.
- Investigate SPF/DKIM Failures: Thoroughly investigate SPF and DKIM failures to determine the root cause and implement appropriate solutions.
Expert view
Expert from Email Geeks clarifies the roles of DKIM and DMARC. DKIM makes a positive assertion that mail was sent by a domain, and its failure is meaningless. DMARC makes negative assertions, and email failing DMARC doesn't automatically mean it's illegitimate. It means the sender wants email to be rejected if there's any doubt about its origin. He also explains that DKIM is not about the content of the message, it is about associating a responsible domain with an email message.
9 Dec 2024 - Email Geeks
Expert view
Expert from Spam Resource highlights issues with indirect mail flow. Forwarding and mailing lists can easily break SPF, since the forwarding server isn't authorized to send mail for the original domain. Even if DKIM is in place, modifications by the forwarder will invalidate the signature, causing DMARC to fail.
31 May 2024 - Spam Resource
What the documentation says
5 technical articles
Legitimate email can fail DMARC, even when properly configured, due to issues in the email's authentication path, particularly related to email forwarding, mailing lists, and the use of third-party senders or multiple domains/shared infrastructure. Forwarding often breaks SPF as the sending server isn't authorized for the original domain. Mailing lists may alter messages, invalidating DKIM. Proper alignment across all sending sources is essential when using multiple domains. The ARC protocol can be used in indirect mail flows to validate the authenticity of forwarded messages.
Key findings
- Forwarding Issues: Email forwarding is a common cause of DMARC failure, as it often breaks SPF.
- Mailing List Problems: Mailing lists can modify messages, invalidating DKIM signatures.
- Third-Party Senders: Misalignment of third-party senders with your domain's SPF and DKIM can lead to DMARC failures.
- Multiple Domains: Lack of proper alignment across multiple domains and sending sources can cause DMARC to fail.
- Authentication Path: Problems in the authentication path (SPF, DKIM) can invalidate DMARC despite proper original setup.
Key considerations
- Implement ARC: Utilize ARC (Authenticated Received Chain) to maintain authentication in indirect email flows.
- Verify 3rd Party Alignment: Ensure that all third-party senders are correctly aligned with your domain's authentication.
- Address Forwarding Issues: Consider solutions to handle email forwarding, such as educating users or using forwarding-friendly authentication methods.
- Align Multiple Domains: Ensure proper SPF/DKIM alignment across all domains and sending sources.
- Monitor Authentication: Consistently monitor email authentication paths to address and correct issues as they arise.
Technical article
Documentation from Valimail.com explains that common causes of DMARC failure include email forwarding, mailing list issues, and problems with third-party senders. Forwarding often breaks SPF, while mailing lists can alter messages, invalidating DKIM. Third-party senders might not be properly aligned with your domain.
20 Jul 2021 - Valimail.com
Technical article
Documentation from Google explains that for indirect email flows, such as forwarding lists, you can make use of ARC (Authenticated Received Chain) to validate mail transfer agents who are forwarding the email have not maliciously altered the original message.
18 Dec 2023 - Google
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