Why is DKIM failing on Gmail, and is Proofpoint causing it?

Marketer from Email Geeks shares that Proofpoint's TAP is likely breaking DKIM with URL rewriting.

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.

Marketer from Email Geeks shares that spam filters and corporate banners will almost always break SPF/DKIM by design.

Marketer from Email Geeks explains that SPF is always susceptible because the sender is unlikely to have the Proofpoint ranges in their SPF record. However, most customers exempt their spam filters' IP ranges from further scanning by the mailbox provider.

Email marketer from SuperUser Forum answers that security appliances or spam filters that modify the email content (like adding footers or scanning/rewriting links) can cause DKIM to break. The original signature doesn't match the altered content, so Gmail flags it as a failure.

Email marketer from EmailOnAcid explains that you can minimize DKIM failures by ensuring you're signing all the right headers, setting up a robust DKIM key size and regularly testing your configuration. It's also important to educate internal teams that use security software to prevent content modification.

Email marketer from EmailDrip mentions that DKIM can fail due to email forwarding, changes made by security services such as Proofpoint, or even by software used to personalize the email. If the contents of the email change, the DKIM signature breaks, leading to failure.

Email marketer from Stack Overflow explains that internal email filtering systems or corporate email policies can modify emails before they reach Gmail. These modifications include adding disclaimers, rewriting URLs, or even changing the subject line, all of which can cause DKIM to fail.

Email marketer from Valimail Blog shares that Proofpoint, and similar security tools, can indeed interfere with DKIM. This typically occurs when these services rewrite URLs or modify email content for security scanning purposes. This modification breaks the DKIM signature, causing it to fail verification at the receiving end, such as Gmail.

Email marketer from Reddit user explains that Proofpoint often breaks DKIM by rewriting URLs for threat analysis. This changes the email's content, making the original DKIM signature invalid when Gmail checks it.

Email marketer from MXToolbox shares that common causes for DKIM failures include: modifications to the email body or headers during transit, incorrect DNS configuration for the DKIM record, and issues with the cryptographic key used for signing. They also highlight that third-party email security services can be a cause.

Marketer from Email Geeks explains that work email setups can rewrite or modify emails by adding disclaimers or rewriting URLs, which will cause DKIM to fail. This often happens when corporate IT uses solutions like Proofpoint in front of GSuite.

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.

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

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.

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.

Documentation from RFC 6376 explains that DKIM relies on the integrity of the message content. Any alteration to the signed parts of the message, whether intentional or unintentional, will cause the DKIM signature verification to fail. This includes changes to headers, body, or any other signed fields.

Documentation from AuthSMTP mentions that DKIM problems often come from third-party email services. These services might modify the email, which then leads to DKIM failing. This is common with email marketing tools and security gateways.

Documentation from Google Workspace Admin Help explains that DKIM failures can occur if the message content is altered in transit. This includes changes by mailing lists, forwarding services, or when a gateway server modifies the email. Also, incorrect DKIM setup, key size, or DNS record errors may cause failures.