Does Microsoft support RFC 8058 list-unsubscribe-post?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 17 Aug 2025
8 min read
For many email senders, a key question in deliverability has been whether Microsoft (Outlook/Microsoft 365) officially supports RFC 8058 List-Unsubscribe-Post. This standard is crucial for implementing one-click unsubscribe functionality, which has become a significant factor in maintaining good sender reputation, especially with recent changes by other major mailbox providers like Gmail and Yahoo.
While Gmail and Yahoo have explicitly mandated RFC 8058 for bulk senders, Microsoft's position has been less overt. However, based on our testing and observations, the answer is yes, Microsoft does support RFC 8058 List-Unsubscribe-Post. This means that if you include the necessary headers in your emails, Microsoft mail clients should process the one-click unsubscribe request as intended.
Understanding the nuances of this support is vital for anyone managing email campaigns, especially given Microsoft's upcoming sender requirements for 2025. While their requirements specify the need for a one-click unsubscribe link, they haven't explicitly singled out RFC 8058 in the same way Google and Yahoo have. Nevertheless, implementing it correctly will ensure compliance and improve your overall deliverability to Microsoft users.
Understanding RFC 8058 and one-click unsubscribe
RFC 8058 is a technical specification that builds upon the older RFC 2369 for the List-Unsubscribe email header. Its primary goal is to standardize a one-click unsubscribe process, making it incredibly easy for recipients to opt out of unwanted emails. Before this, unsubscribing often involved multiple steps, like clicking a link, confirming on a web page, or sending an email to a mailto address. This friction led to more spam complaints.
The core of RFC 8058 lies in the inclusion of two specific headers in your email: List-Unsubscribe and List-Unsubscribe-Post. The List-Unsubscribe header typically contains a URL that the email client can visit. The List-Unsubscribe-Post header provides a URL that the email client should make an HTTP POST request to for a truly one-click unsubscribe without requiring the user to confirm on a landing page. This direct POST request is what enables the seamless, single-action unsubscribe experience.
For more in-depth technical details on the standard, you can refer to the official RFC 8058 specification. Proper implementation ensures that when a user clicks the unsubscribe button offered by their email client (like the one in Outlook), the request is handled automatically and immediately, minimizing the chance of them marking your email as spam.
Unlike Google and Yahoo, Microsoft (or Outlook.com) has not explicitly declared RFC 8058 support as a mandatory requirement for all bulk senders, at least not in the same direct terms. However, they do process these headers and integrate one-click unsubscribe functionality into their user interface. Our recent tests confirm that List-Unsubscribe-Post headers work with Microsoft, even if there might be some minor differences in how the POST request payload is handled.
One common point of confusion among senders is the format of the POST request. Initially, some might assume the unsubscribe ID would be sent in the request body, like this:
Expected POST requestBASH
curl -X POST “https://yourdomain.com/un.cgi” -d “un=some_unsubscribe_id”
However, tests indicate that Microsoft (and other clients) might instead append the unsubscribe ID as a query parameter in the URL, resulting in a request that looks more like this:
Actual POST request observedBASH
curl -X POST “https://yourdomain.com/un.cgi?un=some_unsubscribe_id”
The key takeaway here is that while the format might differ slightly from what some anticipate, the unsubscribe ID is still present and accessible. Senders should configure their unsubscribe endpoints to look for the unsubscribe ID in both the POST body (as a fallback) and, more reliably, as a URL query parameter. This flexibility ensures your system can correctly process the unsubscribe request regardless of the exact method used by the email client.
It's also worth noting that if you only provide a List-Unsubscribe header with an HTTP link (and no mailto version or List-Unsubscribe-Post), Microsoft might still attempt to call that URL, but it could perform a GET request instead of a POST. This is not ideal for one-click functionality, as it often leads to a confirmation page. Therefore, fully implementing RFC 8058 is crucial to ensure a seamless experience and avoid potential deliverability issues.
Microsoft Support: Yes, Outlook (or Microsoft 365) does support RFC 8058 List-Unsubscribe-Post. This ensures one-click unsubscribes work.
Header Requirements: You need to include both List-Unsubscribe and List-Unsubscribe-Post headers in your email. The latter triggers the one-click action.
POST Request Format: Be prepared for the unsubscribe ID to be passed as a URL query parameter, not necessarily in the POST body.
Why one-click unsubscribe matters for deliverability
Implementing RFC 8058 compliant one-click unsubscribe is not just about compliance, it's a critical component of strong email deliverability. When recipients can easily unsubscribe from your emails, they are far less likely to mark your messages as spam. Every spam complaint, especially to major mailbox providers, can negatively impact your sender reputation, leading to more of your emails landing in the junk folder or even being blocked entirely. This is why it’s so important to track your sender reputation using tools like Google Postmaster Tools.
A high spam complaint rate can lead to your IP address or domain being added to a blacklist (or blocklist). Once on a blocklist, your email deliverability can suffer significantly across all mailbox providers, not just Microsoft. It can take considerable effort to get delisted, affecting your business operations and communication. Prioritizing a seamless unsubscribe process is a proactive measure to avoid these serious consequences and maintain healthy sender metrics.
Even if Microsoft hasn't made RFC 8058 explicitly mandatory (yet), adopting it aligns with broader industry best practices for email deliverability. Companies that prioritize user experience and make it easy to opt-out are generally rewarded with better inbox placement and stronger sender reputation. For more details on avoiding blocklists, read our comprehensive guide, An in-depth guide to email blocklists.
Conclusion
Microsoft's new sender requirements, effective early 2025, emphasize the importance of providing a clear and easy unsubscribe option. While they didn't explicitly name RFC 8058, supporting this standard ensures you meet their guidelines for bulk senders. It reinforces good email hygiene, which is beneficial for deliverability and avoiding blocklists (or blacklists). Maintaining a low spam complaint rate and high sender reputation is key to successful email programs, and RFC 8058 plays a direct role in this.
Microsoft has a vested interest in providing a clean inbox experience for its users, and compliant unsubscribe mechanisms are a big part of that. Therefore, even if their documentation isn't as verbose as others on this specific RFC, assume it's a best practice they expect. If you haven't already, ensure your email sending platform is correctly implementing both List-Unsubscribe and List-Unsubscribe-Post to benefit from improved deliverability to Microsoft inboxes.
Views from the trenches
Best practices
Always include both List-Unsubscribe and List-Unsubscribe-Post headers in your bulk emails to ensure proper one-click functionality.
Verify that your unsubscribe endpoint can correctly process IDs passed both in the POST body and as a URL query parameter.
Regularly test your one-click unsubscribe mechanism across different email clients, including Outlook, to confirm it works seamlessly.
Monitor your spam complaint rates in Microsoft's sender tools to identify any issues quickly and maintain a good sender reputation.
Common pitfalls
Assuming the unsubscribe ID will always be in the POST body; some clients may send it as a URL query parameter.
Only implementing List-Unsubscribe with a GET request URL, which often leads to a confirmation page, defeating the one-click purpose.
Not regularly checking for changes in how different mailbox providers process unsubscribe headers, leading to outdated implementations.
Failing to process unsubscribe requests promptly, which can lead to increased spam complaints and harm sender reputation.
Expert tips
Ensure your unsubscribe endpoint responds with a 200 OK status for successful unsubscribes, as this signals completion to the email client.
Consider using a dedicated unsubscribe domain or subdomain to avoid issues if your main sending domain ends up on a blacklist.
Prioritize a smooth unsubscribe experience; it's better for users to unsubscribe easily than to mark your emails as spam.
Keep an eye on Microsoft's official sender documentation for any future explicit requirements regarding RFC 8058.
Expert view
Expert from Email Geeks says that the support for List-Unsubscribe-Post by Microsoft was unclear for a while, even among some Microsoft representatives.
2023-10-23 - Email Geeks
Marketer view
Marketer from Email Geeks says they didn't see an explicit unsubscribe option in the Microsoft user interface, leading them to initially guess that support might be lacking.
Microsoft has a vested interest in providing a clean inbox experience for its users, and compliant unsubscribe mechanisms are a big part of that. Therefore, even if their documentation isn't as verbose as others on this specific RFC, assume it's a best practice they expect. If you haven't already, ensure your email sending platform is correctly implementing both List-Unsubscribe and List-Unsubscribe-Post to benefit from improved deliverability to Microsoft inboxes.
