How does XS4ALL handle DMARC enforcement and DKIM signatures?
Michael Ko
Co-founder & CEO, Suped
Published 22 May 2025
Updated 16 Aug 2025
8 min read
Navigating the complexities of email authentication protocols like DMARC (Domain-based Message Authentication, Reporting, and Conformance) is crucial for ensuring your emails reach their intended recipients. While the core principles of DMARC, SPF (Sender Policy Framework), and DKIM (DomainKeys Identified Mail) are standardized, how individual internet service providers (ISPs) implement and enforce these policies can vary significantly. This variability often dictates whether your legitimate emails land in the inbox or are flagged as spam.
For senders, understanding these nuances is not just theoretical, it directly impacts email deliverability. A robust email authentication setup is your first line of defense against spoofing and phishing attempts, protecting both your brand and your recipients. However, a misconfigured setup or a lack of understanding of ISP-specific enforcement can lead to unexpected delivery failures.
Today, we're going to dive into how one specific European ISP, XS4ALL, approaches DMARC enforcement and its reliance on DKIM signatures. Their policy offers a fascinating case study in how DMARC is applied in the real world and highlights the critical importance of having your email authentication protocols correctly aligned.
DMARC works by instructing receiving mail servers on how to handle emails that claim to be from your domain but fail SPF or DKIM checks. A DMARC record, published in your DNS, specifies a policy like p=none (monitor only), p=quarantine (send to spam), or p=reject (block completely). The effectiveness of these policies, however, depends on the receiving server honoring them. You can read more about DMARC in this Wikipedia article.
XS4ALL, a prominent Dutch internet service provider, has a particular approach to DMARC enforcement that stands out. While many ISPs largely adhere to the DMARC policy as published by the sender's domain, XS4ALL introduces a critical condition related to DKIM signatures. If your email relies solely on SPF for DMARC alignment and lacks a DKIM signature, even if your DMARC policy is set to p=quarantine or p=reject, XS4ALL will effectively ignore this policy and treat it as p=none.
XS4ALL's DMARC policy
XS4ALL's specific DMARC enforcement behavior, which prioritizes the presence of a DKIM signature, has significant implications for email senders. This means that if you're relying solely on SPF to achieve DMARC alignment, your messages may not be subject to the full enforcement of your published DMARC policy at XS4ALL. They are essentially saying that if you want a strong DMARC policy to be enforced, you must also have DKIM.
This stance highlights a broader trend among some ISPs towards preferring or even requiring both SPF and DKIM for robust email authentication. For a comprehensive overview of how these standards work, consider reviewing our guide on SPF, DKIM, and DMARC.
Furthermore, a lack of a DKIM signature with an otherwise DMARC-failing message can negatively impact your domain's reputation with XS4ALL. Even if your message isn't outright rejected, this policy can contribute to a lower sender score, increasing the likelihood of future emails landing in the spam folder (or junk folder, as some providers call it). This underscores the importance of not just having a DMARC policy, but ensuring all underlying authentication mechanisms are properly in place and actively used.
The role of DKIM signatures
DKIM adds a digital signature to your outgoing emails, verifiable by the recipient's mail server using a public key published in your DNS. This signature proves that the email has not been tampered with in transit and that it genuinely originates from your domain. Unlike SPF, which can break during email forwarding, DKIM signatures typically remain valid even when an email is forwarded, making it a more resilient authentication method.
XS4ALL's emphasis on DKIM highlights its recognition of DKIM's robustness. By requiring a DKIM signature for full DMARC enforcement, they are effectively pushing senders towards a stronger, more reliable form of authentication. This means that even if SPF passes, if DKIM is absent and your DMARC policy is to quarantine or reject, XS4ALL will be more lenient, treating it as a p=none policy. This approach protects their users from potential spoofing while also subtly encouraging senders to adopt better authentication practices. Learn how DMARC validates email using DKIM and SPF.
The table below illustrates the key differences and strengths between SPF and DKIM:
Feature
SPF (Sender Policy Framework)
DKIM (DomainKeys Identified Mail)
Primary function
Authorizes sending IP addresses.
Digitally signs email content and headers.
Vulnerability to forwarding
Often breaks if email is forwarded, as the sending IP changes.
Generally resilient to forwarding, signature remains intact.
DMARC alignment
Requires header From domain to match Return-Path domain.
Requires header From domain to match DKIM signature domain.
Complexity
Simpler to set up, but requires keeping IP lists updated.
More complex to set up, requires key generation and rotation.
Implications for senders and deliverability
For email senders, XS4ALL's policy carries clear implications. If your sending infrastructure relies heavily on SPF for DMARC alignment and you send to recipients using XS4ALL, you risk having your DMARC enforcement policy downgraded to p=none at their end if a DKIM signature is not present. This means that even if you intend for DMARC-failing messages to be quarantined or rejected, XS4ALL may still deliver them, potentially undermining your efforts to protect your domain from spoofing. This applies particularly to scenarios involving email forwarding, where SPF validation can often break, as explored in detail in our article on how email forwarding affects SPF, DKIM, and DMARC.
Moreover, not having a DKIM signature can also negatively affect your domain's sender reputation with XS4ALL, even if the emails are still delivered. ISPs constantly evaluate sender reputation, and a consistent lack of DKIM, especially when coupled with DMARC failures, can contribute to a lower trust score. This might result in your emails being more frequently sent to the spam folder, or even added to internal blocklists (also known as blacklists), impacting overall deliverability to a wider audience, not just XS4ALL recipients.
To ensure optimal deliverability, it's essential to implement both SPF and DKIM. While SPF authenticates the sending server's IP address, DKIM provides a cryptographically signed assurance that the email's content has not been altered and that it genuinely came from the claimed domain. Together, they offer a more robust authentication framework that is less susceptible to breaking, particularly important for handling forwarded emails or complex sending scenarios. Our comprehensive guide on DMARC and DKIM best practices offers further insights into proper configuration.
Below are some common challenges senders face with DMARC and DKIM, and solutions to address them:
Challenges
SPF breakage: Email forwarding or mailing lists can cause SPF validation to fail, impacting DMARC.
DMARC policy downgrades: ISPs like XS4ALL may ignore p=reject if DKIM is missing.
Reputation impact: Lack of DKIM can negatively affect sender reputation and lead to spam filtering.
Solutions
Implement DKIM: Always sign your emails with a valid DKIM signature to ensure resilience.
Monitor DMARC reports: Regularly check DMARC aggregate and forensic reports to identify issues.
Gradual DMARC rollout: Start with p=none and move to p=quarantine or p=reject cautiously.
Best practices for DMARC and DKIM
Given XS4ALL's policy, and the general trend among major ISPs like Microsoft and Google to strengthen email authentication requirements, here are some best practices:
Always implement DKIM: Ensure all your sending domains have properly configured DKIM records and that emails are consistently signed. This is crucial for maintaining DMARC policy enforcement and domain reputation.
Start with a cautious DMARC policy: Begin with p=none to gather reports and then gradually move to p=quarantine or p=reject. Our guide on safely transitioning your DMARC policy provides a detailed roadmap.
Monitor DMARC reports closely: Use DMARC reporting tools to gain visibility into how your emails are being authenticated and identify any issues, such as DKIM temperror failures. You can learn more about decoding DKIM temperror issues in our dedicated guide.
A basic DMARC record to start with might look like this:
Ensure DKIM is always configured and active for all sending domains.
Monitor DMARC reports to catch any authentication failures early.
Gradually increase your DMARC policy enforcement from 'none' to 'quarantine' or 'reject'.
Be aware that SPF alone may not be sufficient for full DMARC enforcement with some ISPs.
Common pitfalls
Relying solely on SPF for DMARC, especially with services like XS4ALL.
Ignoring DMARC reports, leading to unaddressed authentication issues.
Setting a strict DMARC policy without proper monitoring or DKIM implementation.
Not understanding how email forwarding impacts SPF and DKIM validation.
Expert tips
Even if SPF passes, a missing DKIM signature can still hurt your domain's reputation with some providers. Always aim for both.
Some senders deliberately omit DKIM with a reject policy to prevent forwarding, but be aware that receiving ISPs may still forward based on their own rules.
Public statements from ISPs about their DMARC handling, like XS4ALL's, are valuable insights. Pay attention to postmaster announcements.
The recipient's policy always takes precedence over the sender's DMARC policy request, allowing ISPs flexibility.
Expert view
Expert from Email Geeks says XS4ALL will not reject for DMARC failure if there is no DKIM signature, effectively treating a quarantine or reject policy as 'p=none' when only SPF is relied upon.
2021-10-04 - Email Geeks
Marketer view
Marketer from Email Geeks says that a lack of DKIM seems to negatively impact domain reputation at XS4ALL.
2021-10-04 - Email Geeks
The importance of robust email authentication
XS4ALL's unique DMARC enforcement policy serves as a powerful reminder that email authentication is not a one-size-fits-all solution. While global standards like DMARC, SPF, and DKIM provide a framework, individual ISPs can and do implement their own interpretations and priorities. Their preference for DKIM signatures highlights the growing recognition of DKIM's resilience and importance in a complex email ecosystem, especially when considering scenarios like email forwarding.
For email senders, the key takeaway is the absolute necessity of a comprehensive authentication strategy. Relying solely on SPF may not be enough to guarantee your DMARC policy is fully honored by all receiving mail servers. Implementing both SPF and DKIM, and ensuring they are correctly aligned with your DMARC record, offers the strongest possible foundation for email deliverability and brand protection.
Proactive monitoring of your DMARC reports is also vital. These reports provide invaluable feedback on how your emails are being authenticated across various ISPs, allowing you to identify and address issues before they significantly impact your sender reputation or deliverability. Staying informed about ISP-specific policies, like that of XS4ALL, empowers you to optimize your email program for maximum inbox placement.
