Summary
DKIM, or DomainKeys Identified Mail, is a critical email authentication standard that allows a recipient mail server to verify that an email was authorized by the domain owner. A common question among email senders, especially when troubleshooting deliverability issues, is whether DKIM signatures are case-sensitive. While DNS records themselves are generally case-insensitive, the specifics of DKIM signature verification, as defined in RFCs, indicate that certain elements should be treated with case sensitivity. Errors often arise not from actual mail server failures, but from how various online testing tools interpret and validate these signatures.
Key findings
- Case sensitivity in tags: According to RFC 6376, the standard that defines DKIM signatures, tags within the signature MUST be interpreted in a case-sensitive manner. This applies to the tags themselves, not necessarily their values in all contexts.
- DNS records are not case-sensitive: While the DKIM signature's internal tags have case sensitivity, the domain names within the DNS records used for DKIM are typically not case-sensitive.
- Tester inconsistencies: Many DKIM validation errors reported by online testers are due to bugs or strict interpretations within the tools themselves, rather than actual failures by major inbox providers like Gmail.
- Domain and identity mismatches: A common tester bug involves failures when the domain in the `d=` tag (signing domain) and the `i=` tag (Agent or User Identifier) differ in case. For more on related issues, see our guide on fixing DKIM body hash failures.
Key considerations
- Trust real-world results: If emails pass DKIM validation with major mailbox providers (like Gmail, Yahoo, Outlook) but fail with a specific online tester, it's often a sign of a bug in the tester, not a misconfiguration on your end. Consider sending a test email and checking the email deliverability tester to view authentication results.
- Consult the RFC: For definitive answers on DKIM protocol specifications, always refer to the official RFC 6376 documentation.
- Standardization is key: Ensure consistency in your DKIM records and email headers to minimize potential issues, even if some systems are more forgiving than others.
- Troubleshooting methodology: When encountering DKIM errors, systematically check your DNS record, email headers, and then compare results across multiple receiving mail servers before concluding a tester is faulty.
What email marketers say
Email marketers often encounter confusing DKIM errors from various online testing tools, leading to concern over their email deliverability. A frequent observation is that while a specific tester flags an issue, actual email sending to major providers like Gmail shows successful DKIM authentication. This discrepancy highlights the importance of distinguishing between a tool's strictness or potential bugs and the practical application of DKIM by recipient mail servers.
Key opinions
- Tester specific issues: Many marketers report that a DKIM failure on one specific tester, like 'About My Email', does not necessarily translate to a failure with actual mailbox providers. This implies the tester might have unique sensitivities.
- Real-world validation: The most reliable test for DKIM (or any authentication) is to send mail to major ISPs (Internet Service Providers) such as Gmail, Outlook, or Yahoo and check their 'Authentication-Results' headers.
- SPF vs. DKIM priority: Some marketers note instances where their email seems to rely solely on SPF authentication, even when DKIM should also be present, suggesting potential issues in how their mail server handles DKIM signing.
- Message modifications: Even subtle alterations to email messages after signing can cause DKIM failures, although this is usually detected by all verifiers. This sensitivity makes DKIM robust against phishing, as explored in articles like Unintentional DKIM failures.
Key considerations
- Cross-platform testing: Always test your DKIM setup by sending emails to multiple major mailbox providers (e.g., Gmail, Outlook, Yahoo) to get a comprehensive view of how your emails are authenticated.
- Examine authentication headers: Learn how to access and interpret the 'Authentication-Results' header in emails received by different ISPs. This header provides definitive information on SPF, DKIM, and DMARC pass/fail status. For more on this, check our guide on how to troubleshoot DKIM failures.
- Verify DKIM keys and selectors: Ensure your DKIM key and selector (used in the DNS record) are correctly configured and match what your email service provider is using. You can find a list of common DKIM selectors here.
Marketer view
Marketer from Email Geeks asked about a surprising DKIM error report. They wanted to confirm that DKIM signatures are not case-sensitive, expressing hope that the issue was not on their end.
Marketer view
Marketer from AutoSPF.com highlighted that DKIM failure occurs when the digital signature of an email cannot be verified by the recipient's server. Such failures can prevent emails from reaching the inbox.
What the experts say
Email deliverability experts frequently weigh in on the nuances of DKIM, especially when it comes to edge cases like case sensitivity and discrepancies between diagnostic tools and live email systems. Their collective wisdom emphasizes reliance on the actual behavior of major inbox providers over the strict, sometimes buggy, interpretations of standalone testers. This practical approach is crucial for real-world email deliverability, preventing unnecessary panic over misleading error reports.
Key opinions
- DNS is not case sensitive: Experts confirm that while DKIM tags might have case-sensitive requirements, the underlying DNS system where DKIM records reside is inherently case-insensitive.
- Tester bugs are common: Several experts suggest that a DKIM failure reported by an online tester, especially if not replicated by major email providers, is likely a bug in the tester itself. This aligns with observations about validation tools sometimes showing errors for valid DKIM records.
- Prioritize authentication-results: The authoritative source for DKIM validation status is the Authentication-Results header provided by the recipient mail server.
- Case sensitivity in specific fields: One specific bug identified by an expert pointed to a tester failing if the domain in the `d=` tag and the `i=` tag differed only in case. This pinpoints a very specific scenario that could lead to false negatives.
Key considerations
- Don't overreact to single tester errors: If you're not seeing DKIM failures across multiple validation points, especially actual recipient mailboxes, a single tester's error should be investigated as a potential bug rather than a critical deliverability issue. This is particularly true if you are experiencing DKIM permerror on specific platforms like Yahoo Mail.
- Seek header analysis: When in doubt, request the raw email headers from a 'real' email delivery to a major provider. These headers contain the definitive authentication results.
- Debugging tester code: Experts with access to tester code can sometimes identify and fix specific bugs, as demonstrated in the provided Slack discussion.
- General DKIM troubleshooting: If you are consistently seeing DKIM failures across multiple providers, consider common issues like why DKIM fails and how to fix it, including DNS record problems or email content modifications.
Expert view
Expert from Email Geeks mentioned that DNS is not case-sensitive, but this fact doesn't guarantee that all email testers are bug-free. He suggested verifying DKIM issues with actual email recipients.
Expert view
Expert from Spam Resource highlighted that precise configuration is vital for DKIM. Any slight deviation, including case differences in certain fields, can lead to validation issues, even if the underlying DNS is forgiving.
What the documentation says
Official DKIM documentation provides the definitive answer to questions regarding case sensitivity and validation processes. While the Domain Name System (DNS) is generally case-insensitive for domain names, the DKIM protocol itself has explicit requirements for how signature tags and their values are to be interpreted. Understanding these specifications is paramount for correct implementation and accurate troubleshooting of DKIM issues.
Key findings
- Tag interpretation: RFC 6376, Section 3.4, explicitly states that tags within the DKIM-Signature header field MUST be interpreted in a case-sensitive manner. If a tag is not recognized, it should be ignored.
- Signature syntax errors: A verifier is instructed to return a PERMFAIL (permanent failure) with a 'signature syntax error' if a required tag is missing or malformed.
- Header field processing: The RFC details how header fields should be processed, including canonicalization rules, which dictate how variations in whitespace or case in header values might affect the calculated hash.
- Canonicalization matters: The chosen canonicalization algorithm (simple or relaxed) impacts how sensitive the signature verification is to changes in whitespace and case within the email's header and body. This directly impacts potential DKIM body hash mismatch failures.
Key considerations
- Strict adherence: While some systems might be more lenient, best practice dictates strict adherence to the RFC's case-sensitivity requirements for DKIM signature tags to ensure universal compatibility and validation. This is particularly relevant when dealing with complex OpenDKIM validation issues.
- RFC 6376 is the source of truth: Any discrepancies found in testers or perceived behavior should always be cross-referenced with the official DKIM Signatures RFC 6376 to confirm the correct implementation.
- Impact on debugging: When troubleshooting, understanding the protocol's explicit rules, including case sensitivity for tags, helps pinpoint the exact cause of validation failures, whether it's a misconfiguration or a faulty tool.
Technical article
IETF Datatracker states in RFC 6376 that tags within a DKIM-Signature header field must be interpreted in a case-sensitive manner. This is a fundamental rule for proper signature validation.
Technical article
IETF Datatracker further explains that if a tag is not understood by the verifier, it must be ignored. However, if a required tag is missing or improperly formed, the verifier should return a PERMFAIL (signature syntax error).
Related resources
6 resources
Related pages
What causes OpenDKIM to incorrectly validate DKIM signatures for emails with long headers?
Why does DKIM show a permerror (bad sig) only on Yahoo Mail?
Is it problematic to use the same DKIM domain and selector for multiple email campaigns, and why do validation tools sometimes show errors for valid DKIM records?
Why is my DKIM body hash failing and how do I fix it?
Why is Microsoft DKIM failing when Gmail passes, and how to fix it?
How to troubleshoot DKIM failures and which tools to use?
Decoding DKIM temperror: what it is and how to fix it
How to fix “DKIM record published no DKIM record found” errors
How to fix DKIM body hash mismatch failures
A simple guide to DMARC, SPF, and DKIM
