Why are emails intermittently failing SPF and DKIM authentication with new Microsoft standards?
Michael Ko
Co-founder & CEO, Suped
Published 21 Jul 2025
Updated 10 Aug 2025
5 min read
Summary
Intermittent SPF and DKIM authentication failures, particularly when sending to Microsoft services, are a common and frustrating issue for email senders. Despite seemingly correct DNS records and passing tests, emails can randomly bounce with SPF=Fail or DKIM=Fail errors. This phenomenon has gained more attention with Microsoft's new, stricter email sending standards, leading many to question the underlying causes of these sporadic rejections.
Key findings
Random Nature: Failures are often inconsistent, with the same email or IP address sometimes passing and sometimes failing SPF or DKIM for the same recipient.
Microsoft's Role: Many issues are attributed to internal Microsoft processes, such as load balancing, internal mail forwarding, or temporary DNS resolution problems.
Authentication Status: Even when SPF fails, DKIM and DMARC might still pass, indicating a specific SPF validation issue. For some senders, the problem is specifically DKIM=Fail.
New Standards: Microsoft's recent stricter authentication requirements for high-volume senders appear to be a contributing factor to the increased visibility of these intermittent issues.
Troubleshooting Difficulty: Diagnosing these random failures is challenging due to their sporadic nature and the lack of consistent, clear error messages beyond a general access denied response.
Key considerations
DNS Resolution: Thoroughly review your SPF DNS resolution tree for potential non-deterministic issues or timeouts. It's important to understand why your emails fail at Microsoft.
Internal Forwarding: Recognize that internal mail forwarding within Microsoft's infrastructure can sometimes inadvertently break SPF authentication.
Continuous Monitoring: Implement robust monitoring of email authentication logs and DMARC reports to detect patterns in these intermittent failures. This helps answer why SPF and DKIM failures occur.
Error Messages: Analyze the specific bounce messages, particularly the '550 5.7.515 Access denied' error, as they often point to authentication issues.
Email marketers frequently encounter baffling intermittent SPF and DKIM authentication failures, especially when targeting Microsoft recipients. These issues often appear random, affecting only a small percentage of emails but causing significant frustration and impact on deliverability. Marketers report that even with seemingly perfect configurations validated by external tools, these sporadic bounces persist.
Key opinions
Random Failures: Many observe seemingly random SPF or DKIM failures, with emails sometimes passing and sometimes failing for the same recipient or domain.
Low Volume Affected: The overall percentage of affected emails is typically low (e.g., 0.2%), yet it can constitute a large proportion of total bounces for well-performing senders.
DNS Consistency: Marketers consistently confirm their SPF records are correctly configured and pass external checks, such as those from AboutMy.Email.
DKIM Specific Issues: Some report issues specifically with DKIM failing while SPF and DMARC pass, indicating a potential signing or processing error.
First Sends Trigger: The intermittent failures can be triggered by the very first sends, not necessarily after a high volume threshold is met.
Key considerations
External Verification Tools: Utilize external tools to get an independent validation of your authentication status, even if internal checks show success.
Bounce Message Analysis: Thoroughly analyze the specific bounce messages for clues, especially the '550 5.7.515 Access denied' error, as this can point to why Microsoft email deliverability is bad.
Monitoring Trends: Continuously track bounce rates and authentication failures over time to identify any emerging patterns or shifts in Microsoft's filtering behavior.
SPF Policy Adjustment: Consider if a softfail SPF policy (~all) might be more appropriate than a hardfail (-all) if intermittent DNS issues are suspected.
Deliverability Mindset: Focus on the overall deliverability rate, understanding that even low percentages of failures can indicate broader issues affecting email deliverability issues.
Marketer view
Email marketer from Email Geeks observes that with new Microsoft standards, SPF fails intermittently despite correct record configuration within SPF includes. They note that the same sending IP can randomly switch between passing and failing SPF for different recipients.
22 May 2024 - Email Geeks
Marketer view
Email marketer from Email Geeks reports that production emails often show SPF aligned in tests, yet Microsoft occasionally rejects them with '550 5.7.515 Access denied' for SPF=Fail, even when DKIM and DMARC pass.
22 May 2024 - Email Geeks
What the experts say
Experts in email deliverability suggest that intermittent SPF and DKIM failures, particularly with Microsoft, often stem from complex underlying issues that may not be immediately apparent from standard DNS checks. They highlight the possibility of internal system quirks within the receiving Mail Transfer Agent (MTA) and the complexities introduced by high-volume sending environments and evolving sender requirements.
Key opinions
DNS Resolution Issues: Intermittent failures can be caused by non-deterministic DNS problems, making SPF resolution unreliable at times.
Microsoft Internal Forwarding: Microsoft's internal mail routing and load balancing can sometimes break SPF authentication due to a historical tendency for internal forwarding to be imperfect.
Cluster-specific Problems: Certain machines within a Microsoft cluster might misprocess SPF on an internal forward, leading to selective failures based on where mail is load balanced.
Temporary Authentication Errors: Issues like DMARC TempError indicate transient problems that prevent full authentication completion. To resolve this, it's key to know how to fix Microsoft Office 365 DKIM.
Beyond Basic Checks: Even if external tools show authentication passing, deeper issues within the receiving infrastructure can still cause intermittent failures.
Key considerations
Trace Headers Analysis: In-depth analysis of email trace headers can reveal internal forwarding paths and help identify where authentication might break within the recipient's system.
Consistent Monitoring: Ongoing monitoring of DMARC reports and bounce logs is essential to detect recurring intermittent issues, providing insights into demystifying SPF TempError.
Root Cause Investigation: Do not assume a correct SPF setup means there are no underlying DNS or internal processing issues that could cause sporadic failures.
Engagement with Receivers: Sometimes, direct engagement with the receiving ISP, such as Microsoft, might be necessary to resolve complex, intermittent authentication problems.
Expert view
Email expert from Email Geeks states that SPF failures at Microsoft indicate a problem during reception, potentially stemming from non-deterministic DNS issues, mail forwarding that breaks SPF, or internal forwarding problems within Microsoft's own infrastructure.
22 May 2024 - Email Geeks
Expert view
Email expert from Wordtothewise emphasizes the importance of DMARC reporting to gain visibility into authentication discrepancies, even when direct troubleshooting proves challenging. This allows for a macro view of email authentication.
24 Apr 2024 - Wordtothewise
What the documentation says
Official documentation and industry insights emphasize that new email sending requirements from major providers like Microsoft demand robust email authentication standards (SPF, DKIM, DMARC) to combat spam and phishing. While these standards are crucial, temporary errors (temperrors) can occur, preventing full authentication. These typically do not lead to immediate rejection unless a stricter policy is enforced.
Key findings
Authentication Mandate: New policies from major email providers, including Microsoft, mandate strong SPF, DKIM, and DMARC authentication for high-volume senders.
Temperrors Explained: SPF and DKIM temperrors are temporary issues that can stop email authentication from completing, as noted by DuoCircle documentation.
Impact of Temperrors: While temperrors don't always cause immediate rejections, they can contribute to DMARC validation failures if not resolved.
Sender Compliance: Senders are expected to meet defined authentication levels, and failure to do so can result in messages being denied, often with specific access denied error codes.
DNS as a Factor: Proper DNS configuration, including the SPF resolution tree, is critical for consistent authentication success and avoiding intermittent issues.
Key considerations
Implement DMARC: Organizations should implement DMARC with monitoring policies to gain visibility into authentication failures, including temperrors. This can also help you decode DKIM temperror.
Monitor DMARC Reports: Regularly review DMARC aggregate reports to identify patterns of authentication issues and temperrors across different receivers.
Ensure Alignment: Beyond simple passing, ensure SPF and DKIM authentication records align with the From domain to satisfy DMARC and improve deliverability.
RFC Understanding: Familiarize yourself with the RFCs for SPF and DKIM, as they define the mechanisms and potential failure modes that can lead to intermittent issues.
Technical article
Microsoft TechCommunity explains that SPF and DKIM temperrors are temporary authentication issues that can prevent completion, though they do not necessarily lead to immediate message rejection. These temporary failures require careful monitoring.
03 Apr 2024 - TECHCOMMUNITY.MICROSOFT.COM
Technical article
DuoCircle documentation defines DMARC TempErrors as temporary authentication problems involving DKIM and SPF standards, which can consequently cause DMARC validation to fail. These issues are often transient and require specific troubleshooting.
