Does the DKIM domain need to align with the List-Unsubscribe domain?
Michael Ko
Co-founder & CEO, Suped
Published 31 May 2025
Updated 17 Aug 2025
8 min read
The world of email deliverability often involves navigating intricate technical requirements. One question that frequently arises, especially with the latest bulk sender guidelines from major mailbox providers, is whether the DKIM domain needs to align specifically with the domain found in the List-Unsubscribe header.
Understanding this relationship is crucial for ensuring your emails not only reach the inbox but also comply with evolving standards designed to protect recipients and reduce spam. The clarification often revolves around the nuances of email authentication protocols like DKIM and DMARC, distinct from specific header content requirements.
Let's dive into the specifics, referencing RFC 8058 and the practical implications for your email program, to determine if this alignment is a hard requirement or a best practice.
DKIM (DomainKeys Identified Mail) is a critical email authentication standard that uses a digital signature to verify the sender's identity and ensure the message hasn't been tampered with in transit. When an email is sent, a cryptographic hash of certain headers and the body is created and encrypted with a private key. This signature is then added to the DKIM-Signature header, which includes a d= tag indicating the signing domain. The receiving server uses this domain to look up a public key in DNS to decrypt and verify the signature.
The List-Unsubscribe header, on the other hand, is designed to provide recipients with a clear and easy way to opt out of email lists. With the latest requirements from major mailbox providers like Google and Yahoo, the presence and proper functioning of a one-click unsubscribe mechanism via the List-Unsubscribe-Post header (as defined in RFC 8058) have become mandatory for bulk senders. You can learn more about these requirements for List-Unsubscribe headers.
For RFC 8058 compliance, the critical point is that the List-Unsubscribe and List-Unsubscribe-Post headers must be included in the DKIM signature. This means they should be listed in the h= tag of your DKIM-Signature to ensure their integrity. If the contents of these headers are altered after signing, the DKIM signature will fail, potentially impacting deliverability. However, this requirement is about *signing* the headers, not necessarily *aligning* the domain within them.
Ensuring List-Unsubscribe Header Integrity
To comply with RFC 8058 and the latest mailbox provider requirements, your email service provider (ESP) or mail server must include the List-Unsubscribe and List-Unsubscribe-Post headers in the list of signed headers in your DKIM-Signature field (the h= tag).
Example of DKIM-Signature with List-Unsubscribe headers
To fully grasp the DKIM domain alignment question, it's essential to understand how email authentication standards interact. DMARC (Domain-based Message Authentication, Reporting, and Conformance) is the key protocol that utilizes both SPF (Sender Policy Framework) and DKIM to verify email authenticity. For a message to pass DMARC, either the SPF domain or the DKIM domain must align with the organizational domain found in the From: header (the address users see).
In the context of DKIM, alignment for DMARC means that the domain specified in the d= tag of the DKIM-Signature must match (either strictly or relaxed) the domain in the From: header. This is a fundamental requirement for DMARC to authenticate the sending domain. It's about tying the cryptographic signature to the domain that the recipient sees as the sender. For more details on this, you can review Google's email sender guidelines.
Crucially, the alignment concept for DMARC applies to the From: header, not typically to other headers like List-Unsubscribe. While the List-Unsubscribe header itself must be covered by a valid DKIM signature, the domain within the unsubscribe URL (e.g., kmail-lists.com in your example) does not need to align with the DKIM signing domain (d=yourdomain.com). The purpose of including it in the DKIM signature is simply to prevent tampering, not to enforce domain consistency between the two fields.
DKIM Signing of Headers
Purpose: To verify the integrity of specific email headers and the message body, ensuring they haven't been altered since signing.
Mechanism: Headers like From:, Subject:, and List-Unsubscribe are listed in the h= tag of the DKIM-Signature and included in the cryptographic hash.
DKIM Domain Alignment for DMARC
Purpose: To ensure that the domain signing the email (DKIM's d=) matches the sender's visible From: domain for DMARC authentication.
Mechanism: The organizational domain in the d= tag is compared to the organizational domain in the From: header. This is a primary factor for Google and Yahoo's new requirements.
Current compliance and future considerations
As of now, neither RFC 8058 nor the current bulk sender guidelines from major mailbox providers explicitly state that the domain used in the List-Unsubscribe URL must align with your DKIM signing domain (d=). The primary focus remains on the List-Unsubscribe and List-Unsubscribe-Post headers being properly DKIM-signed. This ensures that the unsubscribe mechanism is legitimate and hasn't been tampered with. For Yahoo's sender best practices, the emphasis is also on DKIM signing rather than domain alignment for this specific header.
The distinction here is between verifying the authenticity of a header's *presence and content* (via DKIM signing) and verifying the *domain's identity* against the visible sender (via DMARC alignment). The domain within the List-Unsubscribe URL might belong to your ESP or a different service provider, and this is generally accepted as long as the entire header is signed by your legitimate DKIM key.
However, the email ecosystem is dynamic. While not a current requirement, it's not unreasonable to anticipate that mailbox providers could introduce such an alignment requirement in the future. As anti-spam measures evolve, there's a continuous push for greater transparency and consistency across all email identifiers. Proactive senders might consider using the same primary domain or a closely related subdomain for their List-Unsubscribe URLs to build stronger domain reputation and prepare for potential future requirements.
Implementing best practices for deliverability
To maximize your email deliverability, focus on comprehensive authentication. This includes properly configuring SPF, DKIM, and DMARC for your sending domains. Ensure your DKIM d= domain aligns with your From: header for DMARC pass. Explore how to boost email deliverability rates for more technical solutions.
For List-Unsubscribe compliance, ensure that both List-Unsubscribe and List-Unsubscribe-Post headers are present in your emails and, most importantly, are included in your DKIM signature's h= tag. This will satisfy the immediate requirements for one-click unsubscribe. For some contexts, like transactional emails, consider whether these headers are appropriate. Also, be aware of the order of MAILTO and HTTPS in the header.
Regularly monitor your DMARC reports and Google Postmaster Tools to track your authentication rates and domain reputation. This will help you identify any issues promptly, including those related to email blocklists or blacklist entries. Adhering to these best practices will contribute significantly to your overall email deliverability and sender reputation.
Aspect
Requirement
Notes
Header Presence
Both List-Unsubscribe and List-Unsubscribe-Post must be included in emails for bulk senders.
Mandatory for compliance with Google and Yahoo's new requirements.
DKIM Signing
These headers must be included in the h= tag of your DKIM-Signature.
This validates the integrity of the unsubscribe mechanism via RFC 8058.
Domain Alignment
The domain in the List-Unsubscribe URL does not need to align with the DKIM signing domain (d=).
This is a key distinction from DMARC alignment, which focuses on the From: header.
Views from the trenches
Best practices
Ensure the List-Unsubscribe and List-Unsubscribe-Post headers are explicitly included in your DKIM signature's 'h=' tag.
Regularly monitor your DMARC reports for authentication failures, which might indicate issues with DKIM signing.
Use a consistent and reputable domain for all your email sending, even if not strictly required for alignment.
Common pitfalls
Assuming DKIM signing of List-Unsubscribe headers is automatic; always verify your ESP's configuration.
Confusing List-Unsubscribe domain alignment with DMARC's From: header alignment requirements.
Ignoring DMARC reports, which could reveal issues with header integrity or authentication.
Expert tips
Monitor your DMARC reports carefully, as they provide valuable feedback on which headers are being signed and verified.
While RFCs don't mandate List-Unsubscribe domain alignment now, mailbox providers might require it in the future.
Use tools to test your email's compliance with RFCs and current mailbox provider guidelines for authentication and unsubscribe headers.
Marketer view
The DKIM and the List-Unsubscribe domains do not have to align according to current specifications.
2024-06-03 - Email Geeks
Marketer view
The main requirements are a valid DKIM Signature and the inclusion of List-Unsubscribe and List-Unsubscribe-Post headers in the DKIM's signed headers.
2024-06-03 - Email Geeks
Navigating email authentication complexities
While DKIM plays a crucial role in authenticating emails and the integrity of their headers, the domain used in your DKIM signature (d=) does not need to align with the domain in your List-Unsubscribe URL according to current RFCs (like RFC 8058) or present mailbox provider guidelines. The essential requirement is that the List-Unsubscribe and List-Unsubscribe-Post headers are included in the DKIM signature itself (h= tag) to ensure their authenticity.
Focus on robust email authentication for your From: domain via DMARC and ensure all critical headers are signed. While explicit alignment for List-Unsubscribe is not a current mandate, maintaining consistent domain usage where possible is always a smart deliverability practice for future-proofing your email program.
Google and 
Yahoo, the presence and proper functioning of a one-click unsubscribe mechanism via the List-Unsubscribe-Post header (as defined in RFC 8058) have become mandatory for bulk senders. You can learn more about these requirements for List-Unsubscribe headers.
