Why are my authenticated emails to Gmail soft bouncing with a DKIM and SPF fail error?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 May 2025
Updated 16 Aug 2025
7 min read
There's nothing quite as frustrating as seeing your authenticated emails to Gmail soft bounce with a dreaded SPF and DKIM fail error. You've gone through the trouble of setting up your email authentication, expecting smooth delivery, only to be met with bounce messages indicating unauthenticated mail.
This usually comes with an error message like: 550-5.7.26 This mail is unauthenticated, which poses a security risk to the sender and Gmail users, and has been blocked. The sender must authenticate with at least one of SPF or DKIM. For this message, DKIM checks did not pass and SPF check for [your.domain.com] did not pass with ip: [your.ip.address].
It implies that despite your efforts, Gmail's authentication checks are still failing for your domain. This can be confusing and lead to significant deliverability issues.
The good news is that these soft bounces are often fixable. While the error message can seem daunting, a systematic approach to diagnosis and correction can get your emails back into Gmail inboxes.
SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) are fundamental email authentication protocols. SPF specifies which mail servers are authorized to send email on behalf of your domain, while DKIM adds a digital signature to your emails, allowing recipient servers to verify that the message hasn't been tampered with in transit and truly originated from your domain.
When Gmail reports an SPF or DKIM fail, it doesn't necessarily mean your records are completely missing. More often, it indicates a mismatch or misconfiguration. For SPF, the sending IP address might not be listed in your SPF record. For DKIM, the digital signature might be invalid or the DKIM public key published in your DNS might not match the private key used to sign the email.
Beyond simple failure, a crucial concept is DMARC alignment. Even if SPF or DKIM passes individually, if the domains used in these checks don't align with the From header domain (the one your recipients see), the email can still fail DMARC. This is a common reason for authenticated emails being flagged as spam.
Common culprits behind authentication issues
Several factors can lead to SPF and DKIM failures even when you believe they are correctly set up. One of the most common issues involves DNS configuration errors. This could be a typo in your TXT records, an outdated IP address in your SPF record, or an incorrectly published DKIM public key.
Another frequent culprit is email forwarding. When an email is forwarded, the forwarding server often modifies the message headers or sends it from its own IP address, which can break SPF. While DKIM is generally more resilient to forwarding, improper handling by the forwarding server can still invalidate the signature. Google themselves note that forwarded messages can fail SPF.
Additionally, issues can arise from your email sending service or custom configurations. Sometimes, the service you use to send emails might have changed its IP addresses, or you might have recently migrated your DNS, leading to outdated records. It's crucial that your SPF record includes all authorized sending IP addresses, and your DKIM keys are correctly generated and published for every sender you use.
Diagnosing and troubleshooting the problem
The first step in diagnosing these issues is to inspect the email headers of a bounced email. Gmail provides detailed authentication results in the headers, which can tell you precisely where the failure occurred. Look for authentication results for SPF, DKIM, and DMARC.
To view email headers in Gmail, open the email, click the three dots next to the reply arrow, and select Show original. This will display a full breakdown of the email's journey and authentication status. Pay close attention to lines that start with Authentication-Results. This is how you can check authentication results.
Example of a soft bounce authentication result in Gmail headersplain
Authentication-Results: mx.google.com; spf=fail (google.com: domain of user@example.com does not designate X.X.X.X as permitted sender) smtp.mailfrom=user@example.com; dkim=fail header.i=@example.com header.s=default; dmarc=fail (p=none sp=none dis=none) header.from=example.com
After reviewing the headers, verify your DNS records. Ensure your SPF record includes all IP addresses or mechanisms for services that send email on your behalf. For DKIM, confirm that the selector name and public key published in your DNS match what your sending service is using to sign your emails. Even a small change, like a hidden SPF DNS timeout, can cause failures.
Advanced considerations and solutions
To fully resolve soft bounces caused by SPF and DKIM failures, you often need to consider DMARC. DMARC leverages SPF and DKIM to determine if an email is legitimate and provides instructions to recipient mail servers on how to handle emails that fail authentication, such as quarantining or rejecting them. An important aspect of DMARC is that it requires either SPF or DKIM to align with the From header domain.
If your emails are failing DMARC, even if SPF and DKIM pass individually, they will still likely land in spam or be rejected. Implement a DMARC policy that sends you DMARC reports. These reports provide invaluable insights into your email authentication status, helping you pinpoint exactly why emails are failing and which sending sources might be misconfigured. Many services like Kinsta provide guides for fixing DMARC errors.
Maintaining a healthy domain reputation is also critical. Authentication failures negatively impact your sender reputation, making it more likely for your emails to be soft bounced or sent to the spam folder. Regularly monitor your Google Postmaster Tools dashboard for insights into your domain's performance and any authentication errors reported by Gmail.
Getting your emails to Gmail
Soft bounces with SPF and DKIM fail errors to Gmail are a common challenge, but they are not insurmountable. The key is to approach them systematically.
First, carefully inspect your email headers for detailed authentication results. This will provide the specific error messages you need to understand the problem. Next, double-check your DNS records for SPF and DKIM, ensuring they are correctly configured and account for all legitimate sending sources. Pay attention to issues like email forwarding, which can inadvertently break authentication.
Finally, embrace DMARC. Not only does it help enforce your email authentication policies, but its reporting features offer critical visibility into why your emails might be failing. By proactively addressing these issues, you can significantly improve your email deliverability to Gmail and ensure your authenticated messages reach their intended inboxes.
Views from the trenches
Best practices
Always include all legitimate sending IPs and mechanisms in your SPF record, ensuring no authorized sources are missed.
Regularly review your DMARC reports to identify authentication failures and misconfigurations from various sending sources.
Test your email authentication setup thoroughly after any changes to DNS records or sending services to catch issues early.
Common pitfalls
Forgetting to update SPF records when adding new email sending services, leading to SPF validation failures.
Misconfiguring DKIM keys or selectors, resulting in invalid signatures and failed DKIM authentication.
Ignoring the impact of email forwarding on SPF, which can cause legitimate emails to soft bounce or be rejected.
Expert tips
Use a DMARC monitoring tool to simplify the analysis of aggregate and forensic reports, making it easier to pinpoint authentication issues.
If using an ESP, ensure they provide proper DKIM signing and guide you on how to set up SPF and DMARC correctly for your domain.
Consider a DMARC policy of p=quarantine or p=reject once you're confident in your authentication setup to protect against spoofing.
Marketer view
Marketer from Email Geeks says when Google indicates an authentication issue in a bounce message, it is usually accurate.
2023-05-17 - Email Geeks
Marketer view
Marketer from Email Geeks says that sometimes, SPF and DKIM may not even be present in the message headers for authentication to occur.
