Is DKIM signature case-sensitive and what causes DKIM tester errors?
Michael Ko
Co-founder & CEO, Suped
Published 26 May 2025
Updated 19 Aug 2025
7 min read
Email authentication protocols like DomainKeys Identified Mail (DKIM) are crucial for verifying sender identity and combating email fraud. However, setting up and maintaining DKIM can sometimes lead to unexpected errors, particularly when using online testers. A common question that arises is whether DKIM signatures are case-sensitive and if this sensitivity contributes to validation issues.
Understanding the nuances of DKIM, including how it handles case and what frequently triggers errors in testing tools, is vital for ensuring your emails reach their intended inboxes. A misconfigured DKIM record can lead to emails being marked as spam or rejected outright by receiving mail servers, impacting your overall email deliverability.
This guide will clarify the role of case sensitivity in DKIM signatures and delve into the most common reasons why DKIM tester tools might report errors, even when your setup appears correct to recipients.
The short answer is that while DNS itself is generally case-insensitive for domain names and records, certain elements within a DKIM signature and its validation process can be case-sensitive. According to RFC 6376, the specification for DKIM, tags in the DKIM-Signature header must be interpreted case-sensitively, but their values are processed as case-sensitive unless explicitly stated otherwise by the tag's definition. For example, the d= (domain) and i= (Agent or User Identifier) tags are typically treated case-insensitively by email servers when performing alignment checks, but the actual values can sometimes trip up less robust testers if there's a case mismatch.
While DKIM itself has specific rules about case sensitivity, the broader context of DNS records for DKIM keys is less strict. DNS records (like TXT records where your DKIM public key resides) are generally not case-sensitive for the record names or values. This means that selector._domainkey.yourdomain.com will typically resolve the same way regardless of the casing. However, some online DKIM tester tools might implement their parsing or validation logic more strictly than major Mail Transfer Agents (MTAs), leading to false positive errors due to minor case differences in the domain or identifier fields. This is why you might see a test fail, but your emails still pass DKIM validation at Gmail or Yahoo Mail.
Ensuring proper DKIM record formatting
To minimize potential issues, ensure consistency in your DKIM DNS record, particularly for the domain and selector values. While some elements are case-insensitive, adhering to a consistent casing, often lowercase, can help prevent validation discrepancies with various tools. This attention to detail can prevent DKIM body hash failures and other related errors.
Common causes of DKIM tester errors
Beyond case sensitivity, several factors can cause DKIM tester tools to report errors. These tools often perform a comprehensive check of your DKIM setup, from DNS record retrieval to signature validation. One of the most common issues is message modification during transit. DKIM signatures are highly sensitive to any changes in the email's headers or body after the signature is applied. Even a minor tweak, such as a mailing list adding a footer or an antivirus scanner modifying the email, can invalidate the signature, leading to a DKIM failure. This is often seen as a permerror (bad sig) or a body hash mismatch.
DNS configuration errors are another frequent culprit. This includes typos in the DKIM record, an incorrect public key, or the record simply not being published or propagated yet. Misconfigured DKIM selectors, using an expired key, or having an invalid RSA public key can also lead to tester errors. Sometimes, the issue isn't with your setup but with the tester itself, as some tools might be overly strict or have bugs in their validation logic, particularly regarding case sensitivity in less common scenarios.
Temporary errors (often reported as temperror) can also occur due to DNS lookup timeouts or temporary network issues, making it seem like a persistent problem when it's not. These are often transient and may resolve themselves after some time. However, repeated temperror messages could indicate underlying DNS resolution problems that need further investigation.
Diagnosing and resolving DKIM failures
When a DKIM tester flags an error, the first step is to verify if the issue is truly affecting your email deliverability. The most reliable way to do this is by sending a test email to an email account like Outlook.com or Gmail and inspecting the Authentication-Results header. This header provides a definitive status of SPF, DKIM, and DMARC authentication checks performed by the receiving server. If the header shows a DKIM=pass result, then the tester error might be a false positive, possibly due to a bug in the testing tool itself. You can find out more information about troubleshooting DKIM failures and tools to use in our dedicated guide.
Example Authentication-Results HeaderText
Authentication-Results: mx.google.com; dkim=pass (signature was verified) header.d=example.com;
For persistent or genuine DKIM failures, systematic troubleshooting is necessary. Start by meticulously checking your DKIM DNS record for any errors. Even a single character typo or an extra space can invalidate the record, leading to errors like "DKIM record published no DKIM record found." Ensure the v=DKIM1 tag is correctly included and the public key (p=) is accurate and complete. If you suspect an invalid RSA public key error, regenerate your key pair and update the DNS record.
Common DNS misconfigurations
Typos: Small errors in the selector or domain name in the TXT record.
Incorrect key: Public key in DNS doesn't match the private key used for signing.
Missing record: DKIM TXT record is not published or has not fully propagated yet.
Email content changes
Footers/headers: ESPs or security software adding content after signing.
Line breaks: Inconsistent handling of line endings across systems.
Encoding issues: Characters changing during transport affecting hash calculation.
Finally, ensure that your Mail Transfer Agent (MTA) or email service provider is correctly applying the DKIM signature before sending. Issues such as the email being modified by an intermediary server or a Mimecast filter causing body hash failures are common. For decoding DKIM temperror messages, consider network stability and DNS resolver performance as contributing factors. If you're experiencing DKIM failures specifically on Yahoo Mail, it could be related to stricter validation or unique processing by their systems.
Views from the trenches
Best practices
Always check the Authentication-Results header in a real inbox (like Gmail) before concluding a DKIM issue based solely on a tester.
Maintain consistent casing for domain and selector values in your DKIM DNS records to avoid parser inconsistencies.
Implement a DMARC policy with reporting (p=none) to monitor DKIM authentication results directly from receiving mail servers.
Common pitfalls
Assuming a tester error means global DKIM failure; often, it's a false positive specific to the tester's logic.
Not accounting for email content modification by intermediary services, which can break DKIM signatures.
Overlooking small typos or extra spaces in DKIM DNS TXT records that can invalidate the entire signature.
Expert tips
Use a canonicalization method that is less sensitive to whitespace and header reordering for better deliverability.
Regularly rotate your DKIM keys to enhance security and maintain a good sending reputation.
For complex setups, consider consulting RFCs directly to understand the nuances of DKIM specification compliance.
Marketer view
Marketer from Email Geeks says they were seeing an unexpected DKIM error from a tester and hoped DKIM signatures were not case-sensitive, as their Gmail passes.
April 12, 2024 - Email Geeks
Expert view
Expert from Email Geeks says DNS is not case-sensitive, and if failures are not seen with actual recipients, it might be a tester bug, suggesting checking the Authentication-Results header from a real email.
April 12, 2024 - Email Geeks
Final thoughts on DKIM authentication
While the core DKIM specification details specific case sensitivity rules for tags and their values, general DNS resolution tends to be case-insensitive. The primary takeaway is that many DKIM tester errors, particularly those related to case, can be false positives stemming from the tool's strictness rather than an actual problem with your email’s authentication.
The most reliable way to confirm your DKIM setup is to check the Authentication-Results header in emails received by major ISPs. If they report a pass, your DKIM is likely functioning correctly. However, always be vigilant about potential issues such as email content modifications or DNS misconfigurations, as these are genuine causes of DKIM failures.
Regularly monitoring your DMARC reports will provide a comprehensive overview of your DKIM authentication status across various mail servers, giving you the real picture of your email deliverability. This proactive approach helps in quickly identifying and resolving any issues, ensuring your legitimate emails consistently reach the inbox.
Gmail or 
Yahoo Mail.
Outlook.com or 
