Why does DKIM authentication sometimes fail with certain ISPs or receivers like Barracuda and Proofpoint?

Key findings

• Content modification: Security gateways such as Barracuda and Proofpoint often alter email content (e.g., changing URLs for link rewriting), which can invalidate the DKIM signature upon receipt by the end mailbox. This is a primary reason why DKIM authentication sometimes fails for emails passing through these systems, even if the original signature was valid.
• Deployment context: These gateways are typically deployed in enterprise environments rather than for consumer mailboxes. Their configuration can vary significantly between organizations, leading to inconsistent DKIM authentication results.
• DMARC reporting impact: Microsoft Office 365, for example, only includes DMARC reporting if the recipient domain's MX record points directly to O365. If it points to an intermediary gateway (like Proofpoint), DMARC checks might fail at the O365 layer, leading to exclusions from reports even if the primary issue originates earlier in the email flow.
• Interoperability: Consistent failures at specific Message Transfer Agents (MTAs) can indicate an interoperability problem, suggesting that the sender's configuration might technically violate a protocol, even if other MTAs are more forgiving. This requires a deeper technical review to resolve.

Key considerations

• Understand content modification: Investigate whether the specific security gateways are performing URL rewriting or other content alterations that could invalidate DKIM signatures. This is a common cause of DKIM body hash mismatch failures.
• Check ARC support: Authenticated Received Chain (ARC) is designed to preserve email authentication results across multiple hops and intermediate services that modify messages. While not universally adopted, checking for ARC support and proper configuration by your recipient's gateway could help, although trusting ARC results remains an unsolved problem in some contexts.
• Review DMARC policies: Understand how your DMARC policy (e.g., p=quarantine or p=reject) interacts with these authentication failures. A strict policy combined with intermediary modification could lead to unexpected delivery issues, as highlighted by DMARC, SPF, and DKIM alignment failures.
• Technical protocol adherence: If failures are consistent, collaborate with your technical team to ensure that your email sending practices strictly adhere to email protocols, avoiding any technical violations that some MTAs might tolerate but others do not. This proactive approach helps in reducing DKIM temporary errors.

Key opinions

• Inconsistent authentication: Marketers frequently report that DKIM authentication works flawlessly with major ISPs like Gmail and Yahoo, but consistently fails with security providers such as Barracuda and Proofpoint, indicating a specific challenge with these enterprise filters.
• Content alteration impact: A common belief among marketers is that security gateways modify email content (e.g., changing URLs), which then breaks the DKIM signature, leading to authentication failures. This is a primary culprit behind issues with providers like Mimecast, Proofpoint, and Barracuda.
• DMARC policy complications: When DKIM fails due to intermediary services, strict DMARC policies (e.g., p=quarantine) can lead to emails being sent to spam, even for internal or trusted communications, impacting deliverability significantly in B2B scenarios.
• Perceived whitelist solutions: Some marketers suggest that whitelisting IPs of security solutions like Proofpoint is part of the setup process to mitigate these issues. However, this often only applies to internal domains and does not fully resolve problems when sending to external recipients.

Key considerations

• Monitor seed list results: Regularly testing deliverability with seed lists that include recipients protected by Barracuda and Proofpoint is crucial. Consistent failures in these tests indicate a need for deeper investigation into the specific configurations of these security filters. You can use an SPF and DKIM validation checker to assist with this.
• Engage with recipients: When sending to enterprise domains, consider communicating with their IT or security teams to understand their specific email filtering setups and explore potential workarounds or whitelisting opportunities. This can help prevent issues such as Mimecast causing DKIM body hash failures.
• Review email sending infrastructure: Evaluate your own sending infrastructure and configuration to ensure there are no subtle issues that could be causing DKIM to fail when processed by stricter receivers. This may involve examining your DKIM selectors and signing practices.
• Accept partial success: Achieving 100% DKIM authentication across all recipients might be unrealistic given the diversity of receiving systems and security configurations. Acknowledge that a certain percentage of failures might be unavoidable, particularly when dealing with enterprise filters that modify emails, as noted by discussions in the Spiceworks Community.

Key opinions

• Gateways alter content: Security gateways like Proofpoint and Barracuda, positioned in front of destination domains, can alter email content, which often leads to DKIM failures. This modification, such as URL rewriting, breaks the cryptographic signature.
• Pre-modification DKIM verification: Ideally, any sensible filter should perform DKIM verification before modifying the email body. Although many do, the recipient mailbox often receives an already modified copy where the DKIM validation might show as failed post-modification.
• DMARC and forwarding complexities: The interaction of DMARC with email forwarding or gateway processing can be problematic. Even if DKIM initially passes, subsequent modifications by an intermediary before reaching the final mailbox can cause DMARC authentication to fail, leading to emails being flagged as spam.
• ARC as a potential solution: Authenticated Received Chain (ARC) is viewed as a potentially useful mechanism to preserve authentication results through intermediaries, provided the participating filters trust ARC results. However, achieving universal trust in ARC remains an unresolved challenge.
• Interoperability issues: Consistent DKIM failures at particular MTAs suggest an interoperability problem. This often points to the sender's configuration being a technical protocol violation, even if it's accepted by less strict receiving systems. Addressing these violations is key.

Key considerations

• Analyze intermediate filtering: It's essential to understand that security solutions like Barracuda and Proofpoint act as intermediary layers. Their configurations, which might include URL rewriting or other content modifications, are primary culprits for DKIM failures at certain ISPs.
• Leverage DMARC reports for insights: While O365 might exclude DMARC reporting if MX records point to a gateway, other services like Google perform DMARC checks even with modified emails and provide reports. These reports can offer valuable insights into why DMARC authentication fails.
• Address protocol deviations: If DKIM consistently fails at specific MTAs, it's crucial to identify and rectify any technical protocol deviations in your sending methods. This might involve reviewing your message formatting, headers, or other elements that stricter receivers might flag as non-compliant, preventing issues like authentication failures in Yahoo/AOL.
• Explore ARC deployment: While trust is still an issue, exploring and implementing ARC (Authenticated Received Chain) in your email infrastructure can help mitigate DKIM failures caused by forwarding or intermediary processing, as it provides a chain of authentication results. Spam Resource provides details on ARC.

Key findings

• Advanced threat protection mechanisms: Products like Proofpoint's Targeted Attack Protection (TAP) are designed to scan and potentially modify emails to prevent advanced threats. This process, while securing recipients, can inherently break DKIM signatures due to changes in the message body or headers, as documented by Proofpoint's own materials.
• Mimecast's message processing: Mimecast's security gateway performs actions that can modify emails, leading to DKIM failures. Their documentation often provides troubleshooting steps for issues arising from their processing, such as those related to Google Workspace email processing.
• DMARC reporting nuances in Microsoft 365: Microsoft's documentation clarifies that DMARC reports are only generated for messages where the recipient domain's MX record points directly to Office 365. If an email passes through a third-party gateway first, DMARC checks might fail at the O365 layer but won't be reflected in the official DMARC reports, as explained in Microsoft 365 security documentation.
• Role of ARC in preserving authentication: Documentation on ARC (Authenticated Received Chain) highlights its purpose: to provide a way for intermediate mail handlers to assert that they modified an authenticated message, preserving the original authentication results for the final receiver. This helps in scenarios where content modification breaks DKIM.

Key considerations

• Review security gateway configurations: Consult the documentation for specific security gateways like Barracuda and Proofpoint to understand their default behaviors regarding content modification and how these might impact DKIM. Look for options to configure bypasses or exceptions for trusted senders, or explore their handling of ARC.
• Understand DMARC reporting scope: Be aware that DMARC reports might not fully capture all authentication failures if recipient domains use intermediary security solutions. This means you might need alternative methods to detect issues with these specific receivers. Microsoft's documentation offers insight into O365 DMARC reporting.
• Consider ARC for complex mail flows: If your emails frequently pass through multiple intermediaries that modify content, implementing ARC on your sending infrastructure can help preserve authentication across these hops. This is especially relevant when dealing with forwarding services or security gateways that might break DKIM, as seen with Proofpoint's email processing.
• Adhere to best practices for DKIM: Ensure your DKIM setup follows standard best practices, including correct key generation, proper DNS record publication, and consistent signing practices. While intermediaries can cause issues, a robust initial DKIM configuration minimizes potential problems and helps resolve Mimecast's DKIM troubleshooting.