Summary
Successfully delivering emails relies heavily on proper authentication, particularly DKIM, and understanding mechanisms like ARC-Seal which bolster its integrity. Troubleshooting DKIM issues often involves a meticulous review of DNS records, verifying the correct CNAME or TXT entries, and accounting for DNS propagation delays. Beyond initial setup, common problems include public key mismatches, incorrect selectors, or content alterations after signing. When it comes to forwarded messages, the Authenticated-Received-Chain (ARC), identified by the ARC-Seal in headers, becomes vital. This cryptographic signature preserves the original authentication results across intermediary hops, such as mailing lists, ensuring legitimate emails aren't flagged as spam due to what would otherwise appear as broken authentication.
Key findings
- Primary DKIM Setup Issues: Most DKIM verification failures originate from fundamental setup errors, including incorrect or missing CNAME/TXT records in DNS, misconfigured hostnames or selectors, public key mismatches, and insufficient time allowed for DNS propagation.
- Content Alteration Invalidates DKIM: A significant cause of DKIM signature invalidity is the modification of email content or specific headers after the DKIM signature has been applied, leading to body hash mismatches or rendering the signature malformed.
- ARC-Seal Preserves Authentication Integrity: ARC-Seal, a key part of the Authenticated-Received-Chain (ARC) protocol, acts as a cryptographic seal. It verifies the integrity of previous authentication results across intermediary mail hops, confirming that a message's authentication status, including DKIM, was valid at a specific point and has not been tampered with.
- ARC Enhances Deliverability for Forwarded Mail: Implemented by major ISPs like Google, ARC helps preserve email authentication results for forwarded messages and those sent through mailing lists. This mechanism prevents legitimate emails from being incorrectly flagged as spam due to authentication breaks that would otherwise occur from re-transmission.
Key considerations
- Thorough DNS Verification: Always carefully check DNS settings for DKIM, ensuring CNAME or TXT records are accurate, selectors match, and allowing sufficient time for propagation. Using online validators and command-line tools like 'dig' can quickly diagnose common setup issues such as incorrect syntax, missing semicolons, or extra spaces.
- Header and Signature Inspection: When troubleshooting, meticulously inspect email headers for the Authentication-Results and DKIM-Signature fields. Verify that the 'd=' (domain) tag matches the From: domain and the 's=' (selector) tag matches your DNS record. Additionally, check the 'bh=' (body hash) tag for mismatches, which often indicate content alteration after signing.
- Understanding ARC's Role: Recognize ARC-Seal as a crucial component that provides a chain of custody for email authentication results. This cryptographic signature validates the integrity of authentication results, including DKIM, across intermediaries, which is vital for legitimate forwarded emails to pass DMARC checks and avoid spam filters.
- Configuration and Canonicalization: Ensure your sending mail server is correctly configured to sign messages, using the right private key and adhering to proper header and body canonicalization rules. Incorrect canonicalization or accidental content modification after the DKIM signature is applied are common causes of signature invalidation.
What email marketers say
11 marketer opinions
Achieving reliable email deliverability requires meticulous attention to authentication protocols, particularly DKIM, and a clear understanding of supporting mechanisms like ARC-Seal. Troubleshooting DKIM often involves a systematic review of DNS records, verifying the public key's correct publication, and patiently accounting for DNS propagation. Issues frequently arise from incorrect DNS entries, misconfigured selectors, or modifications to email content after the DKIM signature has been applied. Meanwhile, ARC-Seal, as part of the Authenticated Received Chain (ARC), plays a crucial role in maintaining the integrity of email authentication results across multiple mail hops. This cryptographic signature ensures that even forwarded messages, which might otherwise break authentication due to intermediary processing, retain their validated status, thereby significantly improving their chances of passing DMARC checks and reaching the recipient's inbox.
Key opinions
- DNS Configuration is Key for DKIM: Many DKIM implementation issues stem from fundamental DNS record problems, such as using the wrong record type, syntax errors like missing semicolons or extra spaces in the TXT record, incorrect selectors, or simply not allowing sufficient time for DNS propagation across the internet.
- Content Integrity Affects DKIM Validity: DKIM signatures become invalid if email content is modified after the signature is applied. This can lead to 'DKIM signature not valid' errors or body hash mismatches ('bh=' tag), underscoring the importance of maintaining content integrity post-signing.
- ARC-Seal Preserves Authentication Across Hops: ARC-Seal is a critical part of the Authenticated Received Chain (ARC), acting as a cryptographic signature. It validates that the email's authentication results, including DKIM and SPF, were legitimate at previous mail hops and have not been tampered with, creating a trusted chain of custody.
- ARC Enhances Forwarded Email Deliverability: Implemented by major ISPs, ARC is essential for preserving email authentication results for forwarded messages and those sent via mailing lists. This mechanism helps prevent legitimate emails from being incorrectly flagged as spam due to authentication breaks that typically occur during re-transmission.
Key considerations
- Validate DNS Records Rigorously: For DKIM troubleshooting, meticulously verify DNS settings. Common errors include incorrect TXT record types, missing semicolons, extra spaces, or typos in the CNAME/TXT hostname and value. Always ensure the correct selector is used as provided by your email service provider, and account for DNS propagation delays.
- Inspect Email Headers: When diagnosing DKIM failures, thoroughly examine the email headers, specifically the Authentication-Results and DKIM-Signature fields. Confirm that the 'd=' tag, indicating the signing domain, matches your From: domain, and the 's=' tag, the selector, corresponds to your DNS record. A 'bh=' tag mismatch often points to content alteration post-signing.
- Utilize Validation Tools: Leverage online DKIM validators and command-line tools like 'dig' to check your public key's correct publication and accessibility in DNS. These tools can swiftly pinpoint issues such as missing records, incorrect selectors, key mismatches, or syntax errors, accelerating the troubleshooting process.
- Grasp ARC-Seal's Deliverability Impact: Recognize ARC-Seal as a vital cryptographic component within the Authenticated Received Chain (ARC). Its purpose is to verify the integrity of authentication results, including DKIM, across multiple mail hops, which is crucial for legitimate forwarded messages and mailing list traffic to bypass DMARC failures and spam filters.
Marketer view
Marketer from Email Geeks explains that the d=google.com header is listed under the ARC-Seal, which stands for Authenticated-Received-Chain. ARC is implemented by large ISPs like Google and helps preserve email authentication results in any subsequent forwards.
16 Apr 2025 - Email Geeks
Marketer view
Email marketer from Postmark shares that common DKIM implementation issues include incorrect DNS record types, missing semicolons or extra spaces in the TXT record, and not accounting for DNS propagation time. They recommend using online DKIM validators to check the record's validity.
10 Jun 2025 - Postmark
What the experts say
3 expert opinions
Effective email delivery hinges on correctly implemented authentication, especially DKIM, and understanding the role of ARC-Seal. Troubleshooting DKIM issues often begins with identifying if a signature is present in the mail headers. If absent or invalid, common culprits include mismatches between the public key in DNS and the private key used for signing, or issues with message canonicalization and header inclusion. The ARC-Seal, a vital part of the Authenticated Received Chain (ARC) protocol, cryptographically signs prior authentication results and original headers. This creates an immutable record that allows receiving servers to verify the message's authentication integrity even after it has been forwarded by intermediaries, preventing legitimate emails from being incorrectly flagged.
Key opinions
- Missing DKIM Signature: A fundamental DKIM issue is the complete absence of a DKIM signature in the email headers, indicating a failure in the signing process at the sending server or a misconfiguration.
- DKIM Key Mismatch: Incorrect DKIM implementation often stems from a mismatch between the public key published in DNS and the private key used by the sending mail server to sign the email, leading to signature validation failures.
- Canonicalization and Header Issues: DKIM signatures can be invalidated if the email's header or body undergoes improper canonicalization, or if the wrong headers are included or excluded from the signing process, disrupting the integrity check.
- ARC-Seal as Authentication Chain Anchor: The ARC-Seal is a critical component of the ARC protocol, providing an immutable record by cryptographically signing previous ARC-Authentication-Results and ARC-Message-Signature headers, alongside the original message headers.
Key considerations
- Initial Signature Presence Check: When troubleshooting DKIM, the very first step should be to confirm if a DKIM signature is present at all within the email headers. Its absence points to a primary configuration or sending issue.
- Verify Key Synchronization: Crucially, ensure the DKIM public key published in your DNS records precisely matches the private key used by your mail server to sign outgoing messages. A mismatch is a common cause of validation failure.
- Review Canonicalization and Signing Rules: Investigate whether your sending system correctly handles message canonicalization, ensuring the header and body remain unaltered post-signing. Also, verify that the appropriate headers are consistently included or excluded during the signing process.
- Leverage ARC-Seal for Integrity Verification: Understand that ARC-Seal allows receiving mail servers to verify the integrity of the email's authentication chain, especially after forwarding, by providing a verifiable record of its authentication status before intermediary changes.
Expert view
Expert from Email Geeks explains that there is no DKIM signature present on the mail headers.
24 May 2022 - Email Geeks
Expert view
Expert from Spam Resource explains that ARC-Seal is a crucial component of the Authenticated Received Chain (ARC) protocol. It cryptographically signs the previous ARC-Authentication-Results and ARC-Message-Signature headers, along with original message headers, to provide an immutable record of the message's authentication status and any changes made during transit. This mechanism allows receiving mail servers to verify the integrity of the authentication chain, especially after an email has been forwarded by an intermediary that might otherwise break SPF or DKIM authentication.
9 May 2025 - Spam Resource
What the documentation says
5 technical articles
Effective email deliverability often relies on robust DKIM implementation and a clear understanding of supporting protocols like ARC-Seal. Troubleshooting DKIM issues typically involves a rigorous examination of DNS settings, verifying the correct publication of CNAME or TXT records, and accounting for DNS propagation delays. Common pitfalls include incorrect hostnames, public key errors, or content and header alterations that invalidate the signature. Meanwhile, ARC-Seal plays a crucial role by providing a cryptographic chain of custody for authentication results, confirming a message's integrity across intermediary hops. This ensures that even forwarded emails, which might otherwise fail authentication checks, retain their validated status, thereby supporting overall deliverability.
Key findings
- DNS Records are Core to DKIM Issues: Many DKIM verification failures stem directly from DNS configuration issues, including incorrect CNAME or TXT records, misconfigured hostnames, public key errors, or insufficient time for DNS propagation across the internet. These fundamental errors prevent successful signature validation.
- DKIM Failures Due to Content or Header Alteration: Signature verification failures are frequently caused by discrepancies in header or body canonicalization, or content alteration after the DKIM signature is applied. Hash mismatches in the signature indicate that the message has been modified or the signature was malformed during the signing process.
- ARC-Seal Confirms Authentication Chain of Custody: ARC-Seal serves as a cryptographic chain of custody for email authentication results. It confirms that a message's authentication status, including DKIM, was valid at a specific intermediary hop and that the message has not been altered since, providing crucial context for email receivers.
- ARC Enables Forwarded Message Legitimacy: The ARC-Seal, as defined in RFC 8617, specifically signs the ARC-Authentication-Results and ARC-Message-Signature fields, along with select header fields. This is vital for verifying the integrity of the entire Authenticated Received Chain, enabling email receivers to evaluate the legitimacy of messages that might otherwise break authentication due to forwarding or mailing list modifications.
Key considerations
- Leverage Specific Verification Tools: Utilize available tools and cmdlets, such as PowerShell's Get-DkimSigningConfig for Microsoft environments, to programmatically verify your DKIM configuration. These tools can confirm that CNAME records are correctly published and active for all custom domains, providing immediate feedback beyond manual DNS checks.
- Address Canonicalization and Tagging: When DKIM signatures fail, investigate issues related to header or body canonicalization. Discrepancies in how headers are processed or content is handled can lead to hash mismatches. Also, ensure the 'd=' (domain) and 's=' (selector) tags in the DKIM-Signature header precisely match your intended configuration and DNS records.
- Understand ARC-Seal's Integrity Role: Grasp that ARC-Seal is a cryptographic signature crucial for verifying the integrity of the Authenticated Received Chain at each processing hop. This seal confirms that the email's authentication results, including DKIM, were valid at previous intermediaries and that the message has not been altered since, ensuring reliable authentication transfer.
- Verify Public Key Accessibility: A common DKIM troubleshooting step involves confirming that your public key is correctly published and accessible via DNS. If the receiving server cannot retrieve your public key, the signature verification will fail, regardless of other settings. Ensure no DNS propagation delays are hindering this access.
Technical article
Documentation from Google Workspace Admin Help explains that common DKIM verification issues include incorrect CNAME/TXT records, misconfigured hostnames, public key errors, and DNS propagation delays. They advise verifying DNS settings carefully and allowing sufficient time for changes to propagate.
18 Sep 2021 - Google Workspace Admin Help
Technical article
Documentation from Microsoft Learn explains that DKIM implementation involves creating and publishing CNAME records for each selector in DNS. Troubleshooting includes using PowerShell cmdlets like Get-DkimSigningConfig to verify configuration and ensure the correct CNAMEs are published and active for all custom domains.
12 Jun 2023 - Microsoft Learn
How can I troubleshoot DMARC failures and identify the cause of authentication issues?
How do I interpret SpamAssassin DKIM test results and troubleshoot DKIM signature issues?
How do I troubleshoot DMARC failures and potential DKIM replay attacks affecting email deliverability?
How to debug DMARC authentication failure and alignment issues?
How to implement ARC (Authenticated Received Chain) and how does it affect DMARC failures from forwarding?
How to troubleshoot DKIM failures and which tools to use?
