Why do SPF and DKIM failures sometimes occur despite correct setup?
Matthew Whittaker
Co-founder & CTO, Suped
Published 15 May 2025
Updated 16 Aug 2025
8 min read
Email authentication protocols like SPF and DKIM are fundamental for deliverability and preventing spoofing. When you set them up correctly, you expect your emails to pass authentication checks reliably. So, it can be confusing and frustrating to see intermittent SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) failures in your reports, even when everything seems to be in order.
This isn't an uncommon scenario. Instead of a binary pass or fail, a small percentage of failures can occur due to various subtle factors. These issues often stem from how emails are handled after they leave your immediate control, or from specific nuances in how different recipient servers interpret authentication records. Let's explore why these seemingly random failures happen.
SPF (Sender Policy Framework) is designed to prevent email spoofing by allowing domain owners to specify which mail servers are authorized to send email on their behalf. It works by checking the sending server's IP address against a list of authorized IPs published in the sender's DNS (Domain Name System) SPF record. If the sending IP isn't on the list, the SPF check fails.
DKIM (DomainKeys Identified Mail) provides a cryptographic signature to outgoing emails. This signature is then verified by the recipient server using a public key published in your domain's DNS records. DKIM ensures that the email content has not been tampered with in transit and verifies that the email truly originated from the signed domain.
Both protocols are critical for establishing trust in the email ecosystem. When both are correctly implemented, they significantly reduce the likelihood of your emails being marked as spam or rejected. However, their sensitivity to changes during mail delivery can sometimes lead to unexpected failures.
Common causes of SPF failures
One of the most frequent culprits for SPF failures, even with a correct setup, is email forwarding. When an email is forwarded from one server to another, the IP address of the forwarding server becomes the 'last hop' sender. This new sending IP is often not included in your domain's SPF record, causing the SPF check to fail at the final recipient.
Another common issue is having multiple SPF records for a single domain. SPF explicitly states that a domain must have only one TXT record starting with v=spf1. If multiple records exist, the receiving server cannot determine which one to use, leading to a permanent error (PermError) and SPF failure.
The SPF DNS lookup limit is also a frequent cause of failures. An SPF record cannot include more than 10 DNS lookups to evaluate the record. Each a, mx, ptr, and include mechanism counts as a lookup. Exceeding this limit results in a PermError, making SPF validation impossible. It's important to troubleshoot DNS related SPF issues.
Common SPF record errors
Even with a seemingly correct setup, minor errors can lead to SPF failures. These often involve subtle misconfigurations that a quick visual check might miss. Here are a few to be aware of:
Syntax errors: Misspellings, incorrect delimiters, or missing spaces can invalidate the entire record. For example, using ip4:192.0.2.1/32 instead of ip4:192.0.2.1 for a single IP.
Missing quotes: Some DNS providers require the SPF record to be enclosed in double quotes. Without them, the record might be parsed incorrectly.
Invalid mechanisms: Using mechanisms that are not standard SPF (e.g., typos like redirect instead of redirect=).
Example of an SPF record likely to exceed 10 DNS lookupsDNS
DKIM failures often arise when the email message content or headers are altered after the signature is applied. This is a common occurrence with mailing lists, email marketing services, or even some forwarding systems that add or modify elements of the email body or headers, leading to a DKIM body hash mismatch.
Temporary DNS issues can also cause DKIM failures, often reported as a 'temperror'. This means the receiving system was unable to retrieve your DKIM public key from DNS at that specific moment. This can be due to network latency, DNS server overload, or transient lookup problems. While your DKIM setup might be perfectly correct, these external factors can lead to intermittent authentication failures.
Furthermore, an incorrect or expired DKIM key in your DNS record will consistently cause failures. Sometimes, even if the key is correct, an issue with the DKIM selector (the part of the DKIM record that points to the specific public key) can prevent successful validation. It's vital to ensure your DKIM selector names are accurate and widely recognized.
SPF challenges
Forwarding: Changes to the sending IP or 'Return-Path' due to email forwarding.
Multiple records: Presence of more than one SPF TXT record for the same domain.
DNS lookup limits: Exceeding the 10-DNS-lookup limit in the SPF record, resulting in PermError.
DKIM challenges
Message alteration: Any modification to headers or body after signing invalidates the signature.
Key/DNS issues: Incorrect public key, expired key, or temporary DNS retrieval failures (temperror).
Selector mismatch: Discrepancies between the selector used for signing and the one in DNS.
The role of DMARC and alignment
Even if both SPF and DKIM technically pass, your emails can still fail DMARC (Domain-based Message Authentication, Reporting, and Conformance). This is because DMARC introduces the concept of identifier alignment. For DMARC to pass, either SPF or DKIM (or both) must not only pass authentication but also have their respective domains align with the "Header From" domain, which is visible to the end-user.
SPF alignment requires the domain in the 'Return-Path' (Mail From) header to match the "Header From" domain. DKIM alignment requires the domain that signed the email (the 'd=' tag in the DKIM signature) to match the "Header From" domain. If your emails are sent through a third-party service, like an email marketing platform, they might pass SPF or DKIM but use their own domain in the `Mail From` or `d=` tag, leading to an alignment failure.
For example, an email could pass SPF because the sending server's IP is authorized, but if the `Return-Path` domain differs from your visible `From` domain, SPF alignment (and thus DMARC) will fail. Similarly, a DKIM signature might be valid, but if the signing domain doesn't match your `From` domain, DKIM alignment fails. These are common reasons DMARC fails even when everything else seems correct. Understanding DMARC alignment is key.
Scenario
SPF Result
DKIM Result
DMARC Result (p=reject)
Direct send
Pass (Aligned)
Pass (Aligned)
Pass
Forwarded email
Fail (Misaligned)
Fail (Message modified)
Fail (Likely rejected)
Third-party sender (no custom domain)
Pass (Their domain)
Pass (Their domain)
Fail (Misaligned)
Troubleshooting intermittent failures
Diagnosing intermittent failures can be challenging because the issues are not always consistently reproducible. The key is to gather as much data as possible. DMARC reports are invaluable here, as they provide aggregated data on SPF and DKIM authentication results, including details on alignment and reasons for failure.
Check your DMARC aggregate reports from major receivers like Google and Yahoo. These reports (XML files) contain detailed information about authentication results for emails sent from your domain, helping you pinpoint specific IPs or mail streams causing issues. You can find guidance on troubleshooting DMARC reports to assist with this.
It's also important to review how your third-party email service providers (ESPs) handle SPF and DKIM. Some ESPs might require specific configurations or even send from shared IP addresses that could impact your SPF alignment. Always ensure your ESP documentation aligns with your domain's authentication settings. When SPF or DKIM issues arise with specific providers or ISPs, it often requires a deeper dive into their particular handling of email authentication, such as why DKIM may fail at some ISPs but not others, or Microsoft's new standards.
Troubleshooting steps
DMARC reports: Regularly review aggregate reports to identify sources of failure and misalignment. Pay attention to the reported `source_ip` and `auth_results`.
Check email headers: Analyze the `Authentication-Results` header in problematic emails for specific SPF, DKIM, and DMARC outcomes, including 'temperror' or 'permerror'.
DNS configuration: Double-check your SPF and DKIM DNS records for typos, extra spaces, multiple records, or exceeding lookup limits.
Third-party senders: Confirm that all ESPs or transactional email services are correctly configured to align with your domain.
Views from the trenches
Best practices
Maintain meticulous DNS records for SPF and DKIM to prevent common configuration errors.
Consistently monitor DMARC aggregate reports to quickly identify and address authentication failures.
Ensure all third-party email senders are configured to align their authentication with your domain.
Educate your team on email forwarding best practices and their impact on authentication.
Common pitfalls
Overlooking subtle message modifications by mailing lists or forwarding services.
Ignoring the SPF 10-DNS-lookup limit, which can lead to intermittent `PermError` failures.
Having multiple SPF records for a single domain, causing unpredictable authentication results.
Not regularly reviewing DKIM keys for expiration or unintended changes in DNS records.
Expert tips
Use a DMARC monitoring tool to simplify the analysis of authentication reports.
Implement a DMARC policy of `p=none` initially to gather data before enforcing stricter policies.
Validate your SPF and DKIM records regularly using online tools to catch errors early.
Be aware of how email forwarding can break SPF and DKIM, as it's a common issue beyond your direct control.
Expert view
Expert from Email Geeks says that email forwarders frequently cause SPF and DKIM authentication to break because the email's path and headers are altered in transit.
2022-10-08 - Email Geeks
Expert view
Expert from Email Geeks explains that DKIM was never intended to be 100% perfect and random failures can occur due to DNS key retrieval issues or body modifications during forwarding. A 0.1% DMARC failure rate is normal for many clients.
Intermittent SPF and DKIM failures are a nuanced aspect of email deliverability. They often don't point to a fundamental misconfiguration but rather to specific scenarios like email forwarding, subtle message modifications, DNS transient issues, or DMARC alignment challenges with third-party senders.
Proactive monitoring of DMARC reports and meticulous attention to your DNS records are your best defenses. By understanding these common intermittent failure points, you can better diagnose and address issues, ensuring your legitimate emails consistently reach the inbox.
Google and 
Yahoo. These reports (XML files) contain detailed information about authentication results for emails sent from your domain, helping you pinpoint specific IPs or mail streams causing issues. You can find guidance on troubleshooting DMARC reports to assist with this.
