Why does DMARC success rate fluctuate in Google Postmaster Tools despite consistent SPF and DKIM?
Michael Ko
Co-founder & CEO, Suped
Published 13 Aug 2025
Updated 19 Aug 2025
8 min read
Many email senders find themselves scratching their heads when they look at their Google Postmaster Tools (GPT) data. You might see consistent, near 100% success rates for SPF and DKIM, yet your DMARC success rate seems to jump around, sometimes dipping unexpectedly. This can be confusing, especially if your DNS records appear to be perfectly configured.
The key to understanding these fluctuations lies in how DMARC works differently from SPF and DKIM. While SPF and DKIM verify the authenticity of an email based on the sending server and digital signature, DMARC adds an essential layer: alignment. This means the domain in the From header (what your recipients see) must align with the domain that passed SPF or DKIM.
Even with perfect SPF and DKIM setup, DMARC can still fail due to alignment issues or other factors external to your direct sending infrastructure. Understanding these nuances is crucial for maintaining strong email deliverability and ensuring your messages consistently reach the inbox.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds upon SPF and DKIM. It specifies that for a message to pass DMARC, it must pass either SPF or DKIM, and the domain used for that authentication must be aligned with the From header domain. This alignment is often the culprit behind fluctuating DMARC rates, even when SPF and DKIM appear solid.
There are two types of alignment: strict and relaxed. Relaxed alignment is more forgiving, allowing subdomains of the From domain to pass alignment. Strict alignment requires an exact match. If your DMARC record is set to strict alignment and you're sending from subdomains that aren't explicitly aligned, your DMARC success rate can drop. You can learn more about these authentication protocols in a simple guide to DMARC, SPF, and DKIM.
Email forwarding is a common scenario where DMARC can fail despite SPF and DKIM being initially correct. When an email is forwarded, its original SPF record often breaks because the IP address of the forwarding server doesn't match the original sender's SPF record. While DKIM usually survives forwarding if the message content isn't altered, if the DMARC policy relies solely on SPF alignment, the email will fail DMARC upon forwarding.
Another factor is the use of multiple third-party sending services (ESPs). Each ESP needs to be correctly configured for SPF and DKIM, and crucially, for DMARC alignment. If one of your ESPs intermittently fails to achieve DMARC alignment, perhaps due to misconfiguration or changes on their end, your overall DMARC success rate in Google Postmaster Tools will reflect these dips. You may also see your DMARC success rate dropping if certain emails are failing DMARC checks even with correct SPF and DKIM alignment.
This record sets a DMARC policy to 'monitoring only' (p=none), sending aggregate reports to reports@yourdomain.com. Setting up p=noneis a great way to start with DMARC and collect reports without impacting deliverability.
Decoding Google Postmaster Tools data
Google Postmaster Tools provides aggregate data on your domain's authentication success rates, but it's important to understand what this data represents. The DMARC success rate shown in GPT is a percentage of your legitimate mail that passes DMARC checks, versus all mail seen from your domain, including forwarded mail or spoofed mail. This means that a fluctuating DMARC rate in GPT doesn't necessarily indicate a problem with your DMARC record itself, but rather with how your emails are being handled by various recipients and intermediaries.
The data in Postmaster Tools also has a delay, often up to 48 hours, and requires a certain volume of mail to populate. This delay can make it challenging to troubleshoot real-time issues, and low sending volume might result in inconsistent or missing data points, making fluctuations appear more erratic than they are. For more information on using the tool, refer to our Ultimate Guide to Google Postmaster Tools.
While GPT provides a useful overview, it doesn't offer the granular detail needed to pinpoint specific DMARC failures. For that, you need to rely on DMARC aggregate (RUA) reports. These XML reports contain detailed information about DMARC authentication results, including the source IP, domain, and specific reasons for failure. You can gain more insights from understanding and troubleshooting DMARC reports.
Google Postmaster Tools
High-level overview: Provides a summary of SPF, DKIM, and DMARC success rates.
Data delay: Information can be delayed by up to 48 hours, not real-time.
Aggregated data: Combines data from all sources, making it hard to isolate specific issues.
No specific failure reasons: Doesn't tell you *why* DMARC failed for certain emails.
DMARC aggregate reports (RUA)
Detailed insights: Provides granular data on authentication results for each email stream.
Near real-time: Reports are usually sent daily, offering fresher data.
Source identification: Identifies specific IPs and domains that are passing or failing.
Specific failure reasons: Offers insights into why DMARC failures are occurring, such as SPF or DKIM alignment issues.
Common culprits behind DMARC fluctuations
Several factors can contribute to inconsistent DMARC success rates in Google Postmaster Tools, even when SPF and DKIM records appear to be correctly set up. Identifying these common culprits is the first step toward stabilizing your authentication rates.
One primary reason is the sending of mail through third-party services or ESPs (Email Service Providers). While your main domain might have perfect SPF and DKIM, if an ESP sends email on your behalf without proper DMARC alignment, these emails will show up as DMARC failures in Google Postmaster Tools. For example, some ESPs might use their own domain in the Return-Path (for SPF) or d= tag (for DKIM) that doesn't align with your From header domain.
Another often overlooked cause is email forwarding. As mentioned, forwarding can invalidate SPF. If your DMARC policy relies on SPF alignment and an email is forwarded, it may appear as a DMARC failure. This isn't a problem with your initial sending setup, but an inherent challenge with forwarded mail and DMARC. These issues are common reasons for a DMARC drop.
Issue
Impact on DMARC
Example Scenario
Email forwarding
Breaks SPF authentication, leading to DMARC failure if SPF alignment is required.
A customer forwards your newsletter to a friend, causing SPF to fail and DMARC to fail for that specific instance.
Third-party ESPs
May not correctly align SPF/DKIM domains with your From domain.
Your transactional emails sent via SendGrid occasionally fail DMARC if not perfectly configured for alignment, despite SPF/DKIM passing.
Spoofed emails
Show up as DMARC failures, contributing to the overall rate.
Malicious actors sending emails pretending to be from your domain, which fail DMARC.
Volume fluctuations
Lower volumes can make outlier failures more noticeable in percentages.
If you send few emails on a given day, a small number of DMARC failures can skew the success rate significantly.
Actionable steps to stabilize your DMARC success rate
To gain a clearer picture of your DMARC performance and pinpoint the exact reasons for fluctuations, rely on DMARC aggregate (RUA) reports. These detailed XML reports are sent to the email address specified in your DMARC record and provide comprehensive data on all email traffic observed from your domain, including those that pass and fail DMARC. Analyzing these reports is key to understanding why your DMARC success rate is dropping.
Implement a DMARC policy that matches your needs, starting with p=none to monitor traffic without affecting deliverability. Once you've analyzed your aggregate reports and ensured all legitimate sending sources achieve DMARC alignment, you can gradually move to a p=quarantine or p=reject policy. This process helps to safely transition your DMARC policy.
Actively work with your ESPs and internal teams to ensure all email streams are correctly authenticated and, critically, aligned with your domain for DMARC. This involves checking SPF records to include all authorized sending IPs and ensuring DKIM signatures use your domain or a properly delegated subdomain that aligns with your From header. Remember, simply passing SPF and DKIM isn't enough; alignment is key for DMARC success.
Best practices for DMARC stability
Monitor RUA reports: Regularly review DMARC aggregate reports to identify sources of non-compliance.
Ensure alignment: Verify all sending sources align the From header domain with SPF or DKIM passing domains.
Address forwarding: Understand that DMARC failures due to forwarding are often unavoidable and track them separately.
Progress policy slowly: Gradually move from p=none to p=quarantine or p=reject as you gain confidence.
Views from the trenches
Best practices
Always review your DMARC aggregate reports, as they offer detailed insights into authentication results.
Ensure that all your third-party sending services (ESPs) are correctly configured for DMARC alignment, not just SPF and DKIM.
Start with a DMARC policy of `p=none` to monitor traffic and gather data before enforcing stricter policies.
Regularly check for email forwarding issues, which can break SPF and lead to DMARC failures, and account for them in your reports.
Verify that your domain in the 'From' header consistently aligns with your SPF or DKIM authentication domains.
Common pitfalls
Assuming consistent SPF and DKIM success automatically means DMARC success without checking alignment.
Not monitoring DMARC aggregate reports, which are crucial for detailed insights beyond Google Postmaster Tools.
Misinterpreting DMARC failures caused by legitimate email forwarding as issues with your own sending infrastructure.
Moving directly to a `p=quarantine` or `p=reject` policy without thorough monitoring and alignment of all sending sources.
Failing to update DMARC records when adding new third-party sending services or changing existing configurations.
Expert tips
Implement DMARC monitoring tools to automatically parse and visualize XML aggregate reports for easier analysis.
Segment your DMARC reports by source to identify specific ESPs or internal systems causing alignment issues.
Consider using different subdomains for different sending purposes (e.g., transactional, marketing) to isolate and manage their DMARC alignment independently.
Educate your IT and marketing teams on DMARC principles to ensure everyone understands their role in maintaining email authentication.
Regularly check your domain's authentication status using external tools to catch any misconfigurations quickly.
Expert view
Expert from Email Geeks says to get more information on email sources by signing up for DMARC analytics services with user-friendly interfaces to view aggregate reports.
2019-07-29 - Email Geeks
Marketer view
Marketer from Email Geeks says that Google Postmaster Tools may not be 100% accurate, and it's important to check your DMARC reports directly.
2019-07-29 - Email Geeks
Stabilizing your DMARC
Fluctuations in your DMARC success rate within Google Postmaster Tools, even with consistent SPF and DKIM, are usually a sign of DMARC alignment issues or the inclusion of forwarded mail. Google Postmaster Tools offers a valuable high-level view of your email performance, but for granular troubleshooting, the rich detail of DMARC aggregate reports is indispensable.
By actively monitoring these reports and ensuring proper DMARC alignment for all your sending sources, you can stabilize your DMARC success rate, strengthen your domain's sender reputation, and significantly improve your email deliverability. Remember, consistent monitoring and proactive adjustments are key to mastering DMARC and keeping your emails out of the spam folder.
Google Postmaster Tools provides aggregate data on your domain's authentication success rates, but it's important to understand what this data represents. The DMARC success rate shown in GPT is a percentage of your legitimate mail that passes DMARC checks, versus all mail seen from your domain, including forwarded mail or spoofed mail. This means that a fluctuating DMARC rate in GPT doesn't necessarily indicate a problem with your DMARC record itself, but rather with how your emails are being handled by various recipients and intermediaries.
Your transactional emails sent via SendGrid occasionally fail DMARC if not perfectly configured for alignment, despite SPF/DKIM passing.
