Why are emails intermittently failing SPF and DKIM authentication with new Microsoft standards?
Michael Ko
Co-founder & CEO, Suped
Published 10 Aug 2025
Updated 19 Aug 2025
7 min read
Many email senders are encountering a frustrating challenge: emails intermittently failing SPF and DKIM authentication, especially when sending to Microsoft recipients. This issue often manifests as random bounces, even when authentication records seem correctly configured.
The problem isn't always straightforward. You might send an email and see SPF=Pass, only for a subsequent email to a different user, or even the same user, to return SPF=Fail. This randomness makes it particularly difficult to diagnose and resolve.
Microsoft's new sender requirements have amplified the importance of robust email authentication. Understanding why these intermittent failures occur is crucial for maintaining good deliverability and avoiding a Microsoft 550 5.7.515 Access denied error.
Understanding the intermittent SPF and DKIM failures
Email authentication protocols like SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) are foundational for verifying sender identity. SPF validates the sending IP address against a list authorized by the domain owner, while DKIM uses cryptographic signatures to ensure the email content hasn't been tampered with in transit.
When an email intermittently fails SPF or DKIM, it points to a temporary issue, often referred to as a DMARC temperror. These aren't permanent rejections but indicate that the receiving server, in this case, Microsoft, couldn't complete the authentication check at that specific moment. This can happen even if your records are perfectly valid.
Microsoft's strengthened email ecosystem requirements, especially for high-volume senders, mean that authentication failures are now less tolerated. If you send more than 5,000 emails per day to Outlook or Hotmail addresses, your messages must pass SPF, DKIM, and DMARC. Intermittent failures, even if a small percentage, can accumulate and negatively impact your sending reputation. To learn more about this, see Outlook’s new requirements for high-volume senders.
Common culprits behind intermittent authentication issues
One of the primary reasons for intermittent failures often traces back to DNS issues. While your SPF record might be configured correctly, the process of resolving that record can sometimes experience temporary glitches. This might be due to DNS server load, network latency, or transient issues with your DNS provider. If Microsoft's server can't quickly and reliably query your SPF record, it may result in a temporary failure, even if the record itself is valid.
DNS resolution issues
DNS overload: High query volumes or temporary network problems can prevent a quick SPF lookup.
SPF lookup limit: Exceeding the 10-DNS lookup limit in your SPF record can cause failures for some receivers, even if not consistently.
Caching discrepancies: Different DNS resolvers might have varying cache times or stale entries.
Another significant factor, particularly with Microsoft, is their internal email forwarding mechanisms. Reports from the email community suggest that some Microsoft cluster machines may intermittently mishandle internal forwarding, which can inadvertently break SPF authentication. If your email happens to be processed by one of these specific machines, it could result in an SPF failure, even if your setup is impeccable. This is difficult to confirm without internal diagnostic tools, but it's a known historical issue.
Additionally, issues related to DMARC policies or DMARC alignment failures can also contribute to intermittent rejections. While SPF and DKIM might pass independently, DMARC requires at least one of them to align with the 5322.From domain. If this alignment is occasionally broken due to an intermediary or specific mail flow, it can lead to intermittent DMARC failures and subsequent bounces. For more information, see email authentication problems from Microsoft.
Diagnosing and mitigating SPF and DKIM temperrors
Diagnosing intermittent issues requires consistent monitoring. Relying on single test sends isn't enough, as the problem's nature is its inconsistency. Implementing robust DMARC monitoring is essential, as DMARC reports (RUA and RUF) provide aggregate data on how your emails are authenticating across different receivers, including Microsoft. These reports can highlight patterns of failures, even small percentages, that might otherwise go unnoticed. You can learn more about troubleshooting DMARC reports from Google and Yahoo.
When you observe SPF or DKIM failures, inspect the full email headers of the bounced messages. The Authentication-Results header will provide details on why the authentication failed, including specific errors like temperror. If you consistently see temperror, it strongly suggests a DNS-related problem or a temporary network hiccup at the receiver's end. We've got articles on decoding DKIM temperror and demystifying SPF TempError that can help.
While you cannot directly control Microsoft's internal systems, you can ensure your own infrastructure is as robust as possible. This includes using reliable DNS providers with low latency, ensuring your SPF record doesn't exceed the 10-lookup limit, and regularly validating your DKIM keys. Regularly reviewing your Office 365 DKIM and SPF configuration is also a critical step.
Scenario 1: DNS issues
Your DNS server might be under load or experiencing temporary outages, causing SPF lookups to time out. This can lead to random SPF authentication failures even when your record is technically correct.
Solution: Ensure your DNS provider is reliable and has strong uptime. Consider DNSSEC for added security and reliability. Regularly check your SPF DNS timeout.
Scenario 2: Microsoft's internal forwarding
Microsoft's internal mail routing can sometimes lead to SPF or DKIM breakage. An email might be forwarded internally in a way that modifies the header or envelope, causing authentication to fail upon final delivery. This is largely outside your direct control.
Solution: While you cannot fix their internal systems, maintaining a pristine sending reputation and perfectly configured authentication records helps ensure your emails are not unduly penalized for these internal quirks. Consistent monitoring through DMARC reports is key.
Beyond the basics: advanced considerations
For organizations sending a high volume of emails, the new Microsoft standards introduce stricter enforcement. Although intermittent failures may appear random, they can become more frequent or lead to more severe outcomes if your overall sending reputation is not strong. This means focusing on all aspects of deliverability, not just authentication.
It's also worth noting that intermittent failures might impact only certain domains while others using the same IP address remain unaffected. This could point to subtle differences in domain configuration or how different domains are perceived by receiving filters. Even if SPF passes and DKIM fails, or vice-versa, DMARC can still pass if one of the two aligns, but a series of such failures can raise red flags for ISPs.
Sometimes, these authentication errors are triggered without waiting for a specific volume threshold. This suggests that even the first few emails sent can encounter these temporary issues. This underscores the need for real-time monitoring and proactive management of your email authentication to quickly identify and address any underlying problems that may contribute to intermittent failures.
Views from the trenches
Best practices
Ensure SPF records do not exceed the 10-DNS lookup limit to avoid intermittent failures.
Consistently monitor DMARC reports for all domains to catch even small percentages of failures.
Use reliable DNS providers with high uptime and low latency for your email infrastructure.
Regularly validate SPF and DKIM records to ensure they are correctly published and aligned.
Common pitfalls
Relying solely on single test sends for authentication checks, missing intermittent issues.
Ignoring low percentages of SPF/DKIM failures, which can accumulate and harm sender reputation.
Overlooking internal forwarding issues at recipient mail servers, especially with Microsoft.
Assuming perfect configuration means no issues, as temporary network or DNS glitches can occur.
Expert tips
Investigate any SPF or DKIM 'temperror' messages in DMARC reports, as these often point to transient DNS issues.
Be aware that Microsoft's internal mail routing can sometimes inadvertently break SPF on certain clusters.
For intermittent issues, checking the full 'Authentication-Results' header in bounced emails can provide deeper insights.
Consider that random failures for a particular recipient might indicate forwarding on their end, breaking SPF.
Expert view
Expert from Email Geeks says DNS being broken often causes random failures, and it's essential to examine the SPF resolution tree.
2024-05-15 - Email Geeks
Expert view
Expert from Email Geeks says the rejection message indicates SPF is not passing when Microsoft receives the email, which could be due to non-deterministic DNS issues, mail forwarding, or internal Microsoft problems.
2024-05-16 - Email Geeks
Ensuring consistent email authentication
Intermittent SPF and DKIM authentication failures, especially with Microsoft's new standards, are a complex challenge for email senders. While frustrating, they are often a result of transient DNS resolution issues, potential internal forwarding quirks within receiving mail systems, or subtle configuration nuances across domains.
The key to tackling these issues lies in a multi-faceted approach. This includes meticulous attention to your SPF and DKIM records, leveraging DMARC reports for comprehensive insights, and consistently monitoring your email flow for any anomalies. By ensuring your authentication is robust and maintaining a strong sending reputation, you can mitigate the impact of these intermittent failures.
While some factors may be beyond your direct control, such as a receiving server's temporary glitches or internal processing, proactive management of your email infrastructure and continuous observation of authentication results will significantly improve your email deliverability and ensure your messages consistently reach their intended inboxes.
Microsoft 550 5.7.515 Access denied error.
