Does DKIM relaxed alignment cause trouble or is it sub-optimal for deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 23 May 2025
Updated 11 Sep 2025
7 min read
When configuring email authentication protocols, particularly DMARC, a common question arises regarding DKIM alignment. Specifically, whether using relaxed DKIM alignment can cause problems or if it is in some way sub-optimal for email deliverability. This concern often stems from a desire to achieve the strongest possible authentication, which sometimes leads to the assumption that strict alignment is always better.
The reality is that DKIM relaxed alignment is a widely used and often necessary configuration, particularly for organizations that rely on third-party sending services or send emails from various subdomains. While strict alignment offers tighter security by requiring an exact domain match, relaxed alignment provides flexibility without necessarily compromising deliverability. It's about finding the right balance for your specific email sending infrastructure.
Understanding how both strict and relaxed DKIM alignment modes function within the DMARC framework is crucial. It directly impacts how your emails are perceived by receiving mail servers and ultimately, whether they land in the inbox or are flagged as spam (or even rejected). This guide will break down the nuances and help clarify if relaxed alignment is the right choice for your domain's email strategy.
DKIM, or DomainKeys Identified Mail, uses a digital signature to verify the authenticity of an email message, ensuring it has not been tampered with during transit and that it originates from the claimed sender. For DMARC (Domain-based Message Authentication, Reporting, and Conformance) to pass, either SPF or DKIM must be aligned with the From domain. This alignment means that the domain found in the DKIM signature (the 'd=' tag) must relate to the domain in the visible 'From:' header of the email (RFC5322.From).
DMARC defines two modes for this alignment: strict and relaxed. These modes determine how closely the 'd=' domain in the DKIM signature must match the 'From:' header domain. The choice between these modes is critical for email authentication and plays a significant role in your email deliverability.
Generally, having a valid DKIM signature that aligns with your domain is a strong signal to receiving mail servers that your email is legitimate. A failure in DKIM alignment can cause DMARC to fail, even if the DKIM signature itself is technically valid, leading to emails being rejected or sent to spam (also known as a blocklist or blacklist). Ensuring correct DKIM configuration is a fundamental step in building a trustworthy sending reputation.
Relaxed vs. strict DKIM alignment
Strict DKIM alignment
In strict alignment, the 'd=' domain in the DKIM signature must exactly match the domain in the email's 'From:' header. This means if your 'From:' address is newsletter.example.com, the 'd=' tag must also be newsletter.example.com. This offers the highest level of security and precision in domain authentication.
Security: Minimizes spoofing risks by ensuring an exact match.
Relaxed DKIM alignment
With relaxed alignment, the 'd=' domain in the DKIM signature can be a subdomain of the 'From:' header domain. For instance, if your 'From:' address is newsletter.example.com, a 'd=' tag of example.com would still pass alignment. This flexibility is often preferred when using email service providers (ESPs) that sign emails with their own subdomains.
Deliverability: Less likely to cause issues for complex sending architectures.
Many email service providers (ESPs) and complex email setups naturally lead to scenarios where relaxed alignment is beneficial, or even necessary. For example, if an ESP signs emails with someservice.yourdomain.com but the 'From:' header is yourdomain.com, strict alignment would fail, but relaxed alignment would pass. This allows for distributed email sending while maintaining DMARC compliance. You can learn more about these modes in this article on DMARC alignment
The key here is that relaxed alignment doesn't inherently make your email less secure or more prone to deliverability issues, as long as the underlying DKIM authentication passes and there's a DMARC policy in place. It simply broadens the definition of what constitutes an aligned domain. However, it's true that some mailbox providers might favor strict alignment, so it's a factor to consider.
Deliverability impact of relaxed DKIM
The core question is whether DKIM relaxed alignment causes trouble or is sub-optimal for deliverability. Generally speaking, no, it doesn't inherently cause trouble or make deliverability sub-optimal. A properly configured DMARC record with relaxed DKIM alignment is perfectly valid and widely used. It is designed to be flexible enough for various email infrastructures, including those using third-party ESPs. However, there are nuances.
When strict might be preferred
Enhanced Security: For domains with extreme security requirements, strict alignment can provide an additional layer of protection against highly sophisticated spoofing.
Specific Mailbox Provider Requirements: While most major mailbox providers support relaxed alignment, some, like T-Online, have been known to require strict alignment for DKIM, which can lead to deliverability issues if not met.
Domain Reputation: Some experts believe that strict alignment contributes to a stronger domain reputation, although this is often tied more to consistent DMARC enforcement than the alignment mode itself.
One of the primary benefits of relaxed alignment is its compatibility with various email sending services. Many ESPs use their own subdomains for DKIM signing, and without relaxed alignment, DMARC would fail for emails sent through these platforms. This flexibility is essential for businesses that leverage multiple vendors for their email marketing, transactional emails, and other communications. For a deeper dive into DKIM domain alignment failures, including RFC 5322 fixes, see this resource.
Therefore, if your email setup includes subdomains or third-party services, relaxed DKIM alignment is often the pragmatic and recommended choice to avoid unnecessary DMARC failures and ensure smooth email delivery. It's a balance between security and practical implementability for a diverse email ecosystem.
Monitoring and best practices
Regardless of whether you choose strict or relaxed DKIM alignment, continuous monitoring of your email authentication is vital. DMARC reports provide invaluable insights into your email traffic, showing you which emails are passing or failing SPF and DKIM alignment, and why. These reports are the eyes and ears of your email deliverability strategy.
To truly understand your email ecosystem and ensure proper alignment, a robust DMARC monitoring tool is essential. Suped offers the best DMARC reporting and monitoring tool on the market, with the most generous free plan available. Regularly reviewing your DMARC reports on Suped.com allows you to quickly identify any DKIM alignment failures, SPF issues, and potential spoofing attempts. This proactive approach helps maintain a healthy sender reputation and optimal deliverability.
Example DMARC record with relaxed DKIM alignmentDNS
The example DMARC record above uses adkim=r to specify relaxed DKIM alignment. The aspf=r also indicates relaxed SPF alignment. Starting with a p=none policy allows you to gather data without impacting email delivery, giving you time to analyze your reports before moving to a stricter policy like quarantine or reject.
Views from the trenches
Best practices
Always monitor your DMARC reports to identify any unexpected alignment failures for both SPF and DKIM, adjusting configurations as needed.
Start with relaxed alignment for DKIM if you use third-party email service providers or send from multiple subdomains to prevent initial deliverability issues.
Regularly review how your email service provider handles DKIM signing, as some may require specific configurations or settings (e.g., force_dkim_authority in Mailgun).
Ensure your DNS records are correctly published and updated for any new subdomains or sending services to maintain consistent DKIM alignment.
Consider a phased approach: start with relaxed alignment and p=none, then analyze DMARC reports before moving to strict alignment and stricter DMARC policies.
Common pitfalls
Assuming strict DKIM alignment is always superior, potentially causing unnecessary DMARC failures and deliverability problems for complex sending setups.
Not understanding that third-party ESPs might sign with a root domain while you send from a subdomain, leading to strict alignment failures.
Neglecting to monitor DMARC reports, leaving alignment issues undetected and impacting email deliverability and sender reputation over time.
Overlooking vendor-specific settings (like Mailgun's force_dkim_authority flag) that dictate how DKIM keys are generated and aligned for subdomains.
Manually attempting to fix DKIM alignment without consulting the ESP's documentation, leading to incorrect DNS records or conflicting configurations.
Expert tips
Ensure that your DMARC record's 'adkim' tag is set correctly to reflect your desired DKIM alignment mode, whether 's' for strict or 'r' for relaxed.
When troubleshooting DKIM alignment, verify both the 'From:' header domain and the 'd=' tag in the DKIM signature to understand where the mismatch occurs.
Use a tool to check your email authentication headers to see the DKIM 'd=' domain that your ESP is using for signatures.
If using a third-party sender, ensure they provide you with subdomain-specific DKIM keys if you aim for strict alignment.
Don't confuse DKIM alignment with SPF alignment; while both are crucial for DMARC, they operate on different domains (d= for DKIM, Return-Path for SPF).
Expert view
Expert from Email Geeks says that relaxed DKIM alignment typically doesn't cause problems unless specific DMARC configurations are intentionally made to create issues. It's often easier to make DKIM align strictly compared to SPF, which often leads people to configure it that way.
2025-09-04 - Email Geeks
Expert view
Expert from Email Geeks mentioned their platform uses a default setup with an envelope-from unique subdomain on the root, and the only impact is how Google (Gmail) surfaces reputation, requiring senders to add the specific domain to Google Postmaster Tools. This setup hasn't created deliverability issues.
2025-09-04 - Email Geeks
Finding the right balance for your domain
In conclusion, DKIM relaxed alignment is not inherently problematic or sub-optimal for email deliverability. It serves a crucial purpose in supporting diverse email sending infrastructures, particularly those involving subdomains and third-party email service providers. While strict alignment offers tighter security, relaxed alignment provides the necessary flexibility to ensure DMARC compliance without undue complexity.
The key is to understand your specific email environment, the capabilities of your ESP, and to diligently monitor your DMARC reports. With tools like Suped, you can gain clear visibility into your authentication status and make informed decisions to optimize your email deliverability, regardless of whether you opt for strict or relaxed DKIM alignment.
