How to troubleshoot DKIM implementation issues and understand ARC-Seal in email headers?
Matthew Whittaker
Co-founder & CTO, Suped
Published 20 Apr 2025
Updated 14 Aug 2025
5 min read
Summary
Troubleshooting DKIM implementation issues often involves a deep dive into email headers and understanding authentication protocols. This guide explores common pitfalls, such as misconfigured DNS records or server-side signing problems, and clarifies the role of ARC-Seal in maintaining email authenticity across multiple hops. Proper DKIM setup is crucial for email deliverability and ensuring your messages are trusted by receiving mail servers.
Key findings
No DKIM signature: A common issue is the complete absence of a DKIM signature, even after attempted setup, indicating server-side signing has not been enabled or configured correctly.
DNS entry is not enough: Simply adding a DNS TXT record for DKIM is insufficient, the sending mail server must also be configured to generate and attach the DKIM signature to outgoing emails.
ARC-Seal explained: ARC (Authenticated Received Chain) is a protocol that helps preserve email authentication results (SPF, DKIM, DMARC) when an email is forwarded or passes through intermediate mail servers that might alter the message.
Importance of ARC-Seal: It provides a verifiable chain of custody, allowing receiving mail servers to trust the original authentication status even after modifications by intermediaries, reducing false positives for spam.
Key considerations
Server-side configuration: Always confirm that your email sending server or service is actually signing outgoing emails with DKIM, not just that the public key is in DNS.
Header analysis: Meticulous analysis of email headers is essential to diagnose DKIM failures, looking for the DKIM-Signature header and its validity. You can also verify your DMARC, DKIM, and SPF setup.
Domain alignment: For DMARC to pass, the domain in the From header must align with the domain in the d= tag of the DKIM signature.
Web host capabilities: Many web hosting providers offer limited or poorly configured email services, which can complicate DKIM implementation. For more information, you can refer to RFC 8617 on ARC Protocol.
Email marketers often encounter DKIM implementation challenges, particularly when relying on third-party services or web hosts for email sending. Their experiences highlight the importance of thorough verification beyond mere DNS record publication and the often-overlooked role of intermediate servers, especially when dealing with forwarding or mailing lists.
Key opinions
Hosting support struggles: Many marketers find that web hosting companies' support staff are not always well-versed in proper email authentication setup, leading to incomplete DKIM implementations.
DNS-only misconception: There is a common misconception that simply adding a DKIM DNS record is sufficient for signing emails, overlooking the necessary server-side configuration.
Importance of header inspection: Checking email headers post-send is crucial for marketers to confirm if DKIM signatures are actually present and valid, rather than just assuming setup worked.
Learning curve for new protocols: Concepts like ARC-Seal can be initially baffling for marketers, but understanding them becomes essential for advanced deliverability troubleshooting, especially with forwards.
Key considerations
Verify active signing: Always test and verify that emails are indeed being signed by DKIM after configuration, not just that the public key is published.
Educate clients/teams: Marketers may need to educate clients or internal teams on the multi-faceted nature of email authentication, going beyond basic DNS entries.
Understand email flow: Forwards and mailing lists can break traditional SPF and DKIM. This makes ARC a valuable protocol for maintaining authentication signals. You can learn more about ARC (Authenticated Received Chain) in depth.
Patience and persistence: Troubleshooting email authentication issues, especially with uncooperative systems or support, requires patience and a systematic approach. If DKIM fails for specific providers, see why DKIM fails for Outlook.com and Hotmail.com.
Marketer view
Email marketer from Email Geeks explains their client's hosting support advised only a DNS entry for DKIM, but headers showed no change, indicating the server was not actually signing emails.
20 Mar 2019 - Email Geeks
Marketer view
Email marketer from Email Geeks shares their challenge with a client who initially argued that only a DNS entry was needed for DKIM, highlighting common misunderstandings about email authentication.
20 Mar 2019 - Email Geeks
What the experts say
Email deliverability experts emphasize that successful DKIM implementation extends beyond a mere DNS entry, it requires correct server-side configuration to generate and attach signatures. They highlight that the absence of a DKIM signature, even after DNS updates, is a clear indicator of an incomplete setup. Experts also clarify the function of ARC-Seal as a critical mechanism for preserving email authentication across intermediary systems.
Key opinions
Absence of signature: Experts confirm that if no DKIM signature is present in the email headers, the implementation is incomplete, regardless of DNS records.
Beyond DNS: Acknowledging the p= record in DNS is only one part of DKIM, the sending server must actively sign emails.
ARC's role in integrity: ARC is recognized as a vital component for maintaining the integrity of email authentication results through mail relays and mailing lists, preventing legitimate emails from being flagged as spoofed.
Distinguishing ARC from DKIM: Experts help clarify that ARC-Seal headers (with d=google.com or similar) are part of a separate authentication chain for forwarded emails, not a direct DKIM signature issue of the original sender.
Key considerations
Full implementation review: When troubleshooting, experts recommend a holistic review of DKIM, including DNS records, server configuration, and actual outgoing email headers. You can consult a simple guide to DMARC, SPF, and DKIM.
Educating users/clients: Experts often find themselves clarifying common misconceptions, such as the idea that a DNS entry alone suffices for DKIM.
Analyzing ARC headers: Understanding the components of ARC-Seal and ARC-Message-Signature headers can provide crucial context for why an email might pass or fail authentication downstream.
Impact of intermediaries: Recognize that forwarding services and mailing lists can break traditional SPF and DKIM authentication, making ARC an essential part of the modern email ecosystem. For issues like decoding DKIM temperror, consider all factors. More insights can also be found on Spamresource.com.
Expert view
Deliverability expert from Email Geeks confirms that if there is no DKIM signature visible in the email headers, the DKIM implementation is indeed failing.
20 Mar 2019 - Email Geeks
Expert view
Deliverability expert from Email Geeks validates that a client's claim of DKIM working with only a DNS entry is incorrect if the email headers show no signature.
20 Mar 2019 - Email Geeks
What the documentation says
Official documentation and RFCs define DKIM as a method for validating the authenticity of email messages and ARC as a means to preserve authentication results across multiple handling agents. These specifications detail the structure of DKIM signatures, the cryptographic processes involved, and the headers used by ARC to create a verifiable chain of custody for forwarded emails.
Key findings
DKIM signature presence: Documentation specifies that a valid DKIM signature is contained within the DKIM-Signature header field of an email, including tags for version, algorithm, body hash, and signed headers.
DNS record structure: DKIM public keys are published as TXT records in DNS, typically under a subdomain like selector._domainkey.example.com, following a defined format including v=DKIM1 and p=.
ARC chain of custody: RFC 8617 (ARC) outlines how intermediate mail servers add a series of ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results headers to an email, creating a verifiable chain that reflects the email's authentication status at each hop.
ARC for re-authentication: ARC allows a recipient to re-authenticate a message that might otherwise fail SPF or DKIM checks due to modifications by legitimate intermediaries, such as mailing lists or forwarding services. For more on this, see Wikipedia's entry on Authenticated Received Chain.
Key considerations
Header fields and tags: Correct parsing and interpretation of DKIM and ARC header fields and their associated tags (e.g., d=, s=, bh=, cv=) are critical for accurate validation.
Canonicalization methods: DKIM defines canonicalization methods (e.g., simple or relaxed) for headers and body, which dictate how strictly alterations are tolerated without invalidating the signature.
ARC chain validation: Receivers use the ARC-Seal and ARC-Message-Signature headers to validate the entire ARC chain, starting from the outermost seal, to confirm its integrity.
RFC compliance: Adherence to RFCs (like RFC 6376 for DKIM and RFC 8617 for ARC) is essential for interoperability and successful email authentication across the internet. For advanced guidance, explore an advanced guide to email authentication. Also, understanding common DKIM selectors is key.
Technical article
RFC 8617 states that the Authenticated Received Chain (ARC) Protocol allows an intermediate mail transfer agent (MTA) to sign an email's original authentication results, preserving them for subsequent evaluation.
01 Jan 2020 - IETF Datatracker RFC 8617
Technical article
Wikipedia documents that ARC is designed to allow a receiving service to validate an email even when its SPF and DKIM records are rendered invalid by an intermediate server's processing, such as a mailing list.
