Why does DKIM authentication sometimes fail with certain ISPs or receivers like Barracuda and Proofpoint?
Michael Ko
Co-founder & CEO, Suped
Published 8 May 2025
Updated 17 Aug 2025
9 min read
It can be incredibly frustrating to discover that your meticulously configured DKIM authentication is failing for certain recipients, especially with major security gateways like Barracuda and Proofpoint, while seemingly working fine with others such as Gmail or Yahoo. This isn't an uncommon scenario, and I often hear about it from senders striving for perfect email deliverability.
The challenge lies in the complex journey an email takes through various systems before reaching its final destination. While your sending infrastructure might be perfectly aligned, intermediate mail servers and security layers can introduce changes that inadvertently break the DKIM signature. Understanding these dynamics is key to diagnosing and resolving these intermittent failures.
DKIM (DomainKeys Identified Mail) works by attaching a cryptographic signature to the email header. This signature is generated based on a portion of the email's content and header fields. The receiving server then uses the sender's public key, published in their DNS records, to verify this signature. If any part of the signed message, including certain headers or the body, is altered in transit, the signature verification will fail, leading to a DKIM authentication failure. This is why a proper DKIM setup is so critical.
Beyond simple misconfigurations, there are several nuances that can lead to DKIM failures. Sometimes, the server cannot reach the sender's domain DNS zone for lookup, leading to an invalid DKIM signature. Other times, the issue might stem from how the email service provider handles the signing process or even intermittent issues that can be tricky to pinpoint. You can dive deeper into these common pitfalls by exploring seven reasons why DKIM fails.
It's important to remember that DKIM is not just about having a valid signature, but also about the integrity of the message from the moment it's signed until it's verified. Any intermediary that modifies the email content after the DKIM signature is applied will invalidate that signature, regardless of how perfectly it was initially configured.
Gateway behavior and content modification
Security gateways like Barracuda and Proofpoint are typically deployed as a protective layer in front of enterprise destination domains. Their primary function is to inspect incoming email for threats, spam, and phishing attempts. To achieve this, they often perform content alteration, such as rewriting URLs for link protection, adding disclaimers to footers, or even modifying the message body.
This content modification is the leading cause of DKIM failures with these specific systems. When an email is signed with DKIM, the signature is a hash of the original message. If Barracuda (or similar systems) inserts a line break to ensure compliance, as noted in their documentation, this can cause DKIM failure. Similarly, Proofpoint's Targeted Attack Protection (TAP) module, which rewrites URLs, can also break the signature. The original signature no longer matches the altered message, and thus, DKIM authentication fails.
It's a common misconception that if a gateway modifies the email, it must perform the DKIM check beforehand. While many do initially, the copy that reaches the end recipient, or gets processed further by the final mailbox provider (like Google or Microsoft 365), might still show a broken DKIM if it performs a re-verification on the modified message. This means that while the gateway itself might have initially passed DKIM, the final DMARC report from the destination server could still show a DKIM failure.
Gateway
Typical actions leading to DKIM failure
Impact on DKIM
Barracuda Email Security Gateway
URL rewriting (changing links in the email body), adding footers/disclaimers, encoding changes.
Invalidates body hash, leading to DKIM fail if the entire message is signed.
Changes to the message structure or content, causing DKIM verification to fail.
Troubleshooting and resolution strategies
To effectively troubleshoot these issues, start by reviewing your DMARC reports. These reports provide invaluable insights into where DKIM authentication might be failing, which receivers are rejecting or quarantining your mail, and the specific reasons for these failures. While Office 365 might not show failures if its MX record points to a gateway, the DMARC reports themselves will reveal the specific discrepancies. You can use a deliverability testing tool to simulate sending to various providers, including those protected by Barracuda or Proofpoint, and analyze the authentication results.
If you are consistently seeing failures with particular message transfer agents (MTAs), it points to an interoperability problem. I recommend having your technical team investigate if your sending practices contain any technical protocol violations that some MTAs might tolerate but others do not. This could involve examining header canonicalization, body canonicalization, and other DKIM signing parameters to ensure strict compliance with RFCs.
Often, the resolution involves configuring these security gateways to bypass DKIM verification for trusted senders or to perform their content modifications in a way that doesn't invalidate the signature. This usually requires coordination with the recipient's IT department. While you might not have direct control over their setup, providing them with detailed DMARC reports and explaining the impact of their gateway's configuration can facilitate a solution.
Best practices for troubleshooting
DMARC reports: Actively monitor your DMARC reports to identify specific ISPs or receivers consistently reporting DKIM failures. Look for DKIM temperror instances.
Recipient communication: Engage with the IT teams of problematic recipients (those using Barracuda or Proofpoint) to understand their gateway configurations and potentially request whitelisting or adjustments.
Internal validation: Conduct thorough internal DKIM checks using various email testing tools to ensure your own setup is flawless before looking externally.
For specific issues like DKIM failures with Hotmail or Microsoft Office 365, there might be additional factors at play, such as specific Microsoft standards or how their systems process authentication. These scenarios often require a deeper dive into Microsoft's documentation and potential adjustments to your sending practices to ensure compliance.
The role of ARC and DMARC in complex mail flows
The Authenticated Received Chain (ARC) is an authentication standard designed to preserve original email authentication results across multiple hops and intermediate servers. In scenarios where security gateways modify messages, ARC can be a useful solution. It allows intermediate mail servers, such as Barracuda or Proofpoint, to attest to the original authentication status before they make any changes to the email. This way, subsequent receivers can trust the initial authentication, even if the DKIM signature breaks later.
However, the effectiveness of ARC heavily relies on receiving servers honoring and trusting the ARC results. While many major players like Google and Microsoftrecognize ARC, universal adoption and trust are still evolving. Proofpoint, for instance, checks ARC but doesn't sign it themselves, indicating that while they consider the chain, they don't necessarily contribute to it for subsequent hops.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) plays a crucial role here. Even if DKIM fails due to content modification by a gateway, DMARC's policy (e.g., p=quarantine or p=reject) determines the action taken by the receiving server. If the recipient domain has whitelisted the security gateway's IP addresses, the DMARC check might still pass due to the explicit trust relationship, even though the underlying DKIM signature is broken. This means the email will reach the inbox, but DMARC reports might still indicate a DKIM failure. This is often the case in B2B environments where gateways are common.
It's a nuanced situation where control is less about your sending domain and more about the receiving infrastructure. If you're sending to your own domains at Google, you might have the ability to bypass certain checks. However, if you're sending to a gmail.com address, or any consumer mailbox, you have less control over how intermediate security gateways handle your mail before it reaches the final inbox. This highlights the importance of understanding the entire email delivery chain, especially when dealing with advanced enterprise security solutions.
Views from the trenches
Best practices
Actively monitor DMARC reports to spot consistent DKIM failures from specific gateways and addresses.
Work with recipient IT teams to understand their email filtering setup and discuss whitelisting.
Ensure your DKIM implementation adheres strictly to RFC standards to minimize unexpected rejections.
Utilize email deliverability testing tools to simulate deliveries through various security layers.
Common pitfalls
Assuming all DKIM failures are due to your own setup, neglecting intermediary gateway effects.
Not analyzing DMARC reports, missing critical insights into where authentication breaks.
Ignoring the impact of URL rewriting or content modification by recipient security solutions.
Failing to communicate with recipient IT teams about persistent authentication issues.
Expert tips
ARC can help preserve authentication across hops, but its effectiveness depends on receiver trust.
When sending to your own domains protected by a gateway, whitelisting the gateway's IPs can bypass checks.
Consistent DKIM failures with specific MTAs suggest an interoperability issue or technical protocol violation.
Even if DMARC passes due to whitelisting, the underlying DKIM might still be broken due to content alteration.
Expert view
Expert from Email Geeks says Barracuda and Proofpoint are often deployed as a layer in front of the destination domain and can alter content, such as changing URLs, which affects DKIM authentication.
2024-02-05 - Email Geeks
Expert view
Expert from Email Geeks says that Proofpoint's Targeted Attack Protection (TAP) is a common reason why DKIM fails when messages pass through their spam filter.
2024-02-05 - Email Geeks
Ensuring robust email authentication
Navigating DKIM authentication failures, especially with powerful security gateways like Barracuda and Proofpoint, requires a deep understanding of email flow and the potential for content modification. While it can be disheartening to see a perfectly signed email fail authentication, these issues are often a result of intermediary processing rather than a flaw in your initial setup.
By leveraging DMARC reports, understanding the nuances of how these gateways operate, and considering solutions like ARC, you can significantly improve your email deliverability. Remember, consistent communication with the recipient's IT team and continuous monitoring of your email authentication status are crucial steps in ensuring your messages reach their intended inboxes without being flagged or blocked (or blacklisted).
Barracuda and 
Proofpoint are typically deployed as a protective layer in front of enterprise destination domains. Their primary function is to inspect incoming email for threats, spam, and phishing attempts. To achieve this, they often perform content alteration, such as rewriting URLs for link protection, adding disclaimers to footers, or even modifying the message body.
Google or 
Microsoft 365), might still show a broken DKIM if it performs a re-verification on the modified message. This means that while the gateway itself might have initially passed DKIM, the final DMARC report from the destination server could still show a DKIM failure.
Mimecast Email Security Cloud Gateway
