Why does Gmail show DKIM failing when it actually passes?
Michael Ko
Co-founder & CEO, Suped
Published 18 Jul 2025
Updated 18 Aug 2025
6 min read
It can be incredibly frustrating to see a message indicating DKIM failing when all your checks, like those in Google's own header analysis, show it as passing. This often creates confusion and leads senders down rabbit holes trying to fix non-existent problems. We've seen this exact scenario play out for many email senders.
This apparent contradiction usually boils down to how Gmail's user interface interprets DKIM authentication in relation to DMARC alignment, rather than a true failure of your DKIM setup. Let's delve into why this happens and what you can do about it.
DKIM (DomainKeys Identified Mail) is an email authentication standard that uses a digital signature to verify the authenticity of an email message. When an email is sent, the sending mail server signs the message with a private key. The receiving server then uses a public key, published in your domain's DNS records, to verify this signature. If the signature matches, the email's integrity and origin are confirmed.
While DKIM itself verifies the signature, DMARC (Domain-based Message Authentication, Reporting, and Conformance) adds a crucial layer: alignment. DMARC requires that the domain used in the From header (the visible sender address) aligns with the domain that passed DKIM. This alignment can be either strict or relaxed.
In strict alignment, the domains must be an exact match. In relaxed alignment, the From header domain can be a subdomain of the DKIM signing domain. Gmail and other major mailbox providers heavily rely on this DMARC alignment to determine whether an email is legitimate or potentially spoofed.
Why Gmail might report 'FAIL' despite a 'PASS'
The most common reason Gmail might report a DKIM failure, even when the underlying technical check passes, is a DMARC alignment issue. If your From domain doesn't align with the domain specified in the DKIM signature (d= tag) or signing identity (i= tag), Gmail's user interface might present it as a failure to the end-user, even if the DKIM signature itself is valid. This is particularly true if you are using a third-party sending service that signs mail with its own domain.
DKIM verification (Technical Pass)
Signature Validity: The cryptographic signature on the email is correct and matches the public key in your DNS.
Header Integrity: Key parts of the email header and body have not been altered since signing.
SMTP Session: During the actual email transfer (SMTP session), the DKIM check passes successfully, as seen in raw email headers.
Another factor is the display logic within Gmail itself. There have been instances where Gmail's summary table, which provides a simplified overview of authentication results to users, incorrectly flags DKIM as failing due to perceived alignment issues, even when the technical verification passes correctly in the raw message headers. This can happen if their internal checking for Public Suffix List (PSL) based alignment is not functioning as expected, leading to a misleading DKIM fail message.
Common culprits and troubleshooting steps
If you're encountering this issue, begin by examining your raw email headers. Look for the Authentication-Results header. This will show the actual results of the DKIM verification (e.g., dkim=pass). If it shows dkim=pass but the Gmail summary table reports a failure, it's likely an alignment issue or a display bug. You can find steps on how to check these headers in your Gmail account.
Next, review your DMARC record, specifically the adkim tag. If it's set to s (strict), the From domain and the DKIM signing domain must be an exact match. If it's r (relaxed), a subdomain of the DKIM signing domain will pass. Many third-party senders sign with a subdomain or a different domain entirely. To avoid these apparent failures, ensure your From domain aligns with the DKIM domain, or configure your DMARC policy with adkim=r if your setup supports it.
Resolving DKIM-related issues
Align domains: Ensure your From header domain matches or is a subdomain of your DKIM signing domain for adkim=r.
Verify DNS records: Double-check your DKIM DNS record to ensure it's correctly published and propagated.
Minimize modifications: Avoid any changes to the email's content or headers after it has been DKIM signed. Even minor alterations, such as those caused by forwarding (leading to DKIM failing when users forward), can invalidate the signature.
Ensuring consistent authentication with Gmail
For senders using third-party email services, ensure that the service is properly configured to sign emails with your domain or a subdomain you control, and that your DMARC record permits this. If your DMARC policy is set to p=reject or p=quarantine, a perceived DKIM fail in Gmail's UI, even if just a display anomaly, could still impact your deliverability if DMARC also indicates a failure (which it would if alignment failed). This is because DMARC combines both SPF and DKIM authentication results with alignment to determine the email's fate.
Monitoring your DMARC reports from Google and Yahoo is essential. These aggregate and forensic reports provide detailed insights into your email authentication status as seen by recipient servers. They can help you differentiate between a true authentication failure and a mere reporting quirk in a mailbox provider's interface.
In some cases, such as Microsoft's stricter DKIM checks, the problem might be a temporary error (temperror) rather than a persistent failure. These are often transient issues and can resolve themselves. Continuous monitoring and a robust DMARC configuration are your best defenses against email authentication woes and getting caught on an email blacklist (or blocklist).
Conclusion: Navigating DKIM's perceived failures
It's a perplexing situation when your DKIM passes all technical checks, yet Gmail's interface displays a DKIM fail message. This is predominantly an issue of DMARC alignment, where the From domain doesn't align with the DKIM signing domain as required by your DMARC policy. Additionally, some of these reports might be due to temporary display bugs within Gmail's user interface rather than actual authentication problems.
By understanding the nuances of DKIM and DMARC alignment, especially relaxed versus strict modes, and by consistently monitoring your email authentication through DMARC reports, you can ensure your emails are delivered as intended, avoiding unnecessary confusion and potential deliverability issues, even if a UI displays misleading information.
Views from the trenches
Best practices
Always use a consistent 'From' domain that aligns with your DKIM signing domain, especially when using third-party sending services.
Implement DMARC with a relaxed DKIM alignment policy (adkim=r) if you are using subdomains for email sending.
Regularly monitor your DMARC aggregate reports to detect actual authentication failures versus display anomalies.
Perform email deliverability tests using a variety of mailbox providers to ensure consistent authentication results.
Common pitfalls
Misinterpreting Gmail's summary table as definitive proof of a DKIM failure without checking raw headers.
Using a strict DKIM alignment policy (adkim=s) when sending through third-party services that sign with a different domain.
Ignoring the difference between a technical DKIM pass and a DMARC alignment pass.
Making unnecessary changes to DNS records or sending infrastructure based on misleading UI reports.
Expert tips
Check for any intermediary services that might modify email headers or content after DKIM signing, as this can invalidate the signature.
If using Google Workspace, ensure your DKIM record is properly set up and published according to their guidelines.
Be aware that DMARC reports show the precise technical status, which is often more accurate than simplified UI summaries.
Remember that email forwarding can often break DKIM signatures, leading to authentication failures for forwarded messages.
Expert view
Expert from Email Geeks says that Gmail reports DKIM failures when the DKIM domain does not align with the RFC 5322 From domain, even if the DKIM signature is technically valid.
2025-02-07 - Email Geeks
Marketer view
Marketer from Email Geeks says there appears to be a bug in Gmail's summary table regarding alignment checks, sometimes claiming misalignment when the domains are indeed aligned.
