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

Email marketer from an Email Marketing Forum explains that sometimes DKIM failures are seen as less important if SPF is passing. However, for maximum deliverability, both SPF and DKIM should pass. They suggest checking DNS records for errors.

Email marketer from Mailhardener shares that ARC (Authenticated Received Chain) is designed to address DKIM failures in forwarding scenarios by preserving authentication results across multiple hops. ARC allows receiving servers to trust the original authentication status even if the DKIM signature is broken by an intermediary.

Marketer from Email Geeks explains that Mimecast can "explode" messages, which can break DKIM authentication.

Email marketer from StackOverflow responds that DKIM failures can happen if there are character encoding problems. If the message changes from UTF-8 to something else during transmission, it will change the hash and the signature will fail.

Email marketer from Reddit's r/emailmarketing forum shares that email security gateways like Proofpoint and Barracuda often modify email content (e.g., adding disclaimers or rewriting URLs) which then invalidates the DKIM signature. The user recommends whitelisting sender IPs if possible.

Marketer from Email Geeks explains that part of the setup process involves whitelisting the Proofpoint IPs to prevent DKIM alignment issues.

Marketer from Email Geeks shares that TAP (Targeted Attack Protection from Proofpoint) can cause DKIM failures when passing through a spam filter.

Email marketer from Quora answers that Gmail and other major ISPs are now much stricter with DMARC policies. If DKIM fails and SPF isn't properly aligned, messages are much more likely to go to spam or be blocked, especially if the sending domain has a strict DMARC policy.

Email marketer from EasyDMARC responds that common reasons for DKIM failures include incorrect DNS configuration, modifications to the message content during transit, and issues with the DKIM key itself. They emphasize the importance of regularly monitoring DKIM reports to identify and resolve these issues.

Email marketer from Valimail explains that DKIM failures often occur in indirect mail flows (e.g., forwarding lists) due to modifications made by intermediaries. This breaks the DKIM signature and can lead to deliverability issues.

Marketer from Email Geeks explains that Barracuda and Proofpoint are usually deployed as a layer in front of the destination domain and can alter content, potentially affecting DKIM authentication.

Expert from Email Geeks states that any sensible filter should be doing DKIM verification _before_ they modify the body.

Expert from Email Geeks suggests ARC might be a useful solution to DKIM authentication issues, especially if filters trust the ARC results.

Expert from Email Geeks suggests that consistent DKIM failures at particular MTAs may indicate an interoperability problem, where the sender might be using practices that some MTAs accept but technically violate protocol.

Expert from Spam Resource responds that DKIM can fail when emails are forwarded, especially through mailing lists, because the forwarding process often modifies the message headers or body, thus invalidating the original DKIM signature. ARC can help solve this.

Expert from Word to the Wise explains that authentication failures, including DKIM, are often a symptom of deliverability problems. Understanding the cause, and if the cause can be changed is important. Often an ESP support team may be able to help solve DKIM related problems.

Documentation from Microsoft Learn explains that DKIM failures can occur due to email forwarding, where the forwarder modifies the message headers or body, invalidating the DKIM signature. It highlights the importance of ARC (Authenticated Received Chain) to preserve authentication results across multiple hops.

Documentation from RFC Editor (RFC6376, the DKIM standard) explains that DKIM signatures can fail verification if the message body or headers are modified in transit. Changes made by intermediaries, such as adding disclaimers or converting character encodings, will invalidate the signature.

Documentation from DMARC.org clarifies that DKIM failures can lead to DMARC failures if the message doesn't align with SPF or other authentication mechanisms. DMARC relies on DKIM and SPF to verify the sender's identity, and a DKIM failure can cause the message to be rejected or marked as spam depending on the DMARC policy.