Summary
DKIM failures in Gmail are frequently caused by alterations to email content during transit. Proofpoint and similar third-party security solutions often rewrite URLs, add disclaimers, or modify content for security scanning purposes, invalidating the DKIM signature. Corporate email policies, internal filtering systems, and email forwarding can also alter content, leading to DKIM failures. Incorrect DKIM setup, DNS configuration errors, and issues with cryptographic keys are additional contributing factors. SPF is similarly susceptible to breakage. Regular testing of email authentication and education on security software impact are crucial for preventing DKIM failures.
Key findings
- Third-Party Security Interference: Third-party security solutions like Proofpoint often modify email content, causing DKIM failures.
- Content Modification: Alterations to email body, headers, or content after signing invalidate the DKIM signature.
- Corporate Email Policies: Corporate policies introducing disclaimers or URL rewriting can cause DKIM failures.
- Configuration Issues: Incorrect DKIM setup, DNS configuration, or key issues contribute to DKIM failures.
- Forwarding Impacts: Email forwarding servers can modify headers or body, causing DKIM signature invalidation.
Key considerations
- Regular Testing: Regularly test DKIM setup to detect failures caused by third-party services or internal policies.
- Education on Security Software: Educate internal teams about the impact of security software on email authentication.
- Proper DKIM Configuration: Ensure robust DKIM key size, correct DNS record setup, and proper header signing.
- Monitor Third-Party Impact: Monitor the impact of third-party services on email content to prevent DKIM failures.
- DKIM-Aware Forwarding: Use DKIM-aware forwarding services to prevent signature invalidation during email forwarding.
What email marketers say
12 marketer opinions
DKIM failures in Gmail often stem from alterations to email content during transit. These alterations can be due to corporate email policies, third-party security services like Proofpoint (which rewrites URLs for threat analysis), or even email personalization software. SPF is also susceptible to these modifications. Modifying email content invalidates the DKIM signature, which then causes the authentication check in Gmail to fail.
Key opinions
- Third-Party Interference: Security solutions such as Proofpoint, Mimecast and corporate spam filters frequently rewrite URLs and modify email content for security purposes, leading to DKIM failure.
- Content Modification: Any changes to the email body, headers, or attached content after DKIM signing will invalidate the signature and cause failure.
- Corporate Policies: Corporate email policies that add disclaimers or alter subject lines can also invalidate DKIM.
- SPF Susceptibility: SPF is also susceptible to breakage by intermediate servers, though DKIM is designed to be more resilient in these cases.
Key considerations
- Regular Testing: Regularly test your DKIM setup to ensure third-party services or internal policies aren't causing failures.
- Key Size and Configuration: Ensure you have a robust DKIM key size and that your DNS records are configured correctly.
- Educate Internal Teams: Educate internal teams about the impact of security software on email authentication to prevent unintended modifications.
- Content Integrity: Minimize unnecessary modifications to email content during transit to maintain DKIM validity.
Marketer view
Marketer from Email Geeks shares that Proofpoint's TAP is likely breaking DKIM with URL rewriting.
1 Mar 2022 - Email Geeks
Marketer view
Email marketer from dmarc.org writes that SPF, DKIM and DMARC are the main ways of authenticating email, however, due to the nature of SPF, changes in message content are more likely to cause authentication failure, so DKIM is often chosen as a better method.
14 Jan 2022 - dmarc.org
What the experts say
2 expert opinions
DKIM failures in Gmail are often caused by third-party email security solutions, such as Proofpoint, that modify email content, add disclaimers, or rewrite URLs for security scanning purposes. This modification breaks the DKIM signature, leading to authentication failure. Therefore, regular testing of email authentication is crucial to identify if third-party services are invalidating the signing process.
Key opinions
- Third-Party Security Solutions: Third-party email security solutions, including Proofpoint, are a common cause of DKIM failures.
- Email Content Modification: Modifications to email content, such as adding disclaimers or rewriting URLs, break the DKIM signature.
Key considerations
- Regular Testing: Implement regular testing of email authentication to ensure third-party services are not invalidating the signing process.
- Authentication Monitoring: Monitor email authentication results to quickly identify and address DKIM failures caused by third-party services.
Expert view
Expert from SpamResource shares that third-party email security solutions can often cause DKIM failures. These solutions, including Proofpoint, may modify email content, add disclaimers, or rewrite URLs for security scanning, which breaks the DKIM signature.
17 Mar 2024 - SpamResource
Expert view
Expert from Word to the Wise shares that it is always best practice to test your authentication on a regular basis to ensure that third party services which may be altering your email, such as Proofpoint or similar, are not invalidating the signing process
27 Aug 2024 - Word to the Wise
What the documentation says
5 technical articles
DKIM failures in Gmail, as highlighted by various documentation sources, are primarily attributed to alterations of email content during transit. These alterations can occur due to a number of reasons including modification by mailing lists, forwarding services, gateway servers, email marketing tools, or security gateways such as Microsoft's Exchange Online Protection (EOP). These services and processes often modify the email headers or body. Erroneous DKIM setup, incorrect key size or DNS record errors may also lead to failures. Since DKIM relies on the integrity of the message content, any changes, intentional or otherwise, invalidates the DKIM signature.
Key findings
- Message Alteration: Altering email content in transit is a primary cause for DKIM failures.
- Forwarding Issues: Email forwarding, when the forwarding server modifies the headers or body, leads to DKIM failures.
- Third-Party Services: Third-party email services, including email marketing tools and security gateways, can modify email content and trigger DKIM failures.
- Configuration Errors: Incorrect DKIM setup, key size, or DNS record errors contribute to DKIM failures.
Key considerations
- Maintain Message Integrity: Ensuring the integrity of the email message throughout the delivery process is crucial to prevent DKIM failures.
- Configuration Review: Regularly review DKIM setup, key size, and DNS records to ensure they are correctly configured.
- DKIM-Aware Forwarding: Use DKIM-aware forwarding services to prevent signature invalidation.
- Monitor Third-Party Impact: Monitor the impact of third-party services and tools on email content to avoid unintended DKIM failures.
Technical article
Documentation from Microsoft explains that Exchange Online Protection (EOP) can affect DKIM if it modifies the email content. EOP is designed to protect against spam and malware, and in doing so, it might rewrite URLs or add disclaimers, invalidating the original DKIM signature.
22 Feb 2025 - Microsoft
Technical article
Documentation from DMARC Analyzer mentions that email forwarding is a common cause of DKIM failures. When an email is forwarded, the forwarding server often modifies the email headers or body, which invalidates the DKIM signature. This is particularly problematic when the forwarder is not DKIM-aware.
7 Nov 2021 - DMARC Analyzer
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