How can I identify all Microsoft owned inboxes to exclude them from email audiences?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Aug 2025
Updated 16 Aug 2025
8 min read
Managing email audiences for optimal deliverability can be a complex task, especially when dealing with major mailbox providers like Microsoft. Many email senders try to exclude Microsoft-owned inboxes from their audiences to manage sender reputation or execute specific rewarming strategies. The challenge often lies in identifying all of them, beyond the obvious ones like @hotmail.com or @outlook.com. Even after diligently removing these known domains, you might still observe send volume reported in Microsoft SNDS, indicating that other Microsoft-controlled inboxes are still present in your audience.
The key to successfully excluding Microsoft-owned inboxes is to understand the broader ecosystem of domains they manage, which extends far beyond the most commonly recognized ones. This includes a variety of international domains and even custom domains hosted on Microsoft's infrastructure. Accurately identifying these can significantly impact your deliverability rates and sender reputation with Microsoft accounts.
Uncovering lesser known Microsoft domains
When you think of Microsoft email, domains like hotmail.com, live.com, msn.com, and outlook.com immediately come to mind. These are indeed core consumer domains. However, Microsoft's reach extends globally, encompassing a multitude of country-specific top-level domains (TLDs) and regional variations.
For example, you might encounter domains like hotmail.co.uk, outlook.ca, or even webtv.net, which are all managed by Microsoft. A simple query using wildcards on your email database, like searching for '%hotmail%' or '%outlook%', can help uncover many of these international variations that you might be missing.
Beyond these, there are also numerous custom domains that utilize Microsoft's hosting services, particularly for business and academic purposes. While Microsoft Sender Network Data Services (SNDS) primarily reports on consumer-facing domains, some B2B domains hosted by Microsoft might still contribute to your overall send volume and impact your deliverability metrics, though they might not directly show up in the same manner as consumer domains. This is why a comprehensive approach is necessary, not just relying on a pre-compiled list. For specific strategies on what Microsoft domains to exclude in your segmentation, further exploration is often needed.
Leveraging MX records for precise identification
One of the most effective methods to identify all domains hosted by Microsoft, including those less obvious, is to perform MX (Mail Exchanger) record lookups. An MX record specifies the mail server responsible for accepting email messages on behalf of a domain. By examining these records, you can determine if a domain's email is routed through Microsoft's infrastructure.
Microsoft's free webmail services often point to MX records ending in olc.protection.outlook.com, while domains hosted on Office 365 (now Microsoft 365) typically point to mail.protection.outlook.com. There are also specific endpoints for academic or government clients, such as those pointing to mail.outlook.com. By taking a list of domains you send to and performing MX lookups, you can build a more accurate exclusion list.
This method is more robust than relying solely on domain name patterns, as it identifies domains actively using Microsoft's mail infrastructure, regardless of their public-facing address. It’s particularly useful for identifying B2B domains that use Microsoft 365 for their email services.
Example MX lookup using dig (Linux/macOS) or nslookup (Windows)bash
dig MX example.com
nslookup -type=MX example.com
After running an MX lookup, look for results that contain Microsoft-specific strings. This will indicate that the domain's email is handled by Microsoft. For example, if you see .protection.outlook.com or .mail.outlook.com in the MX records, then the domain is hosted by Microsoft. You can use this information to decide whether to exclude these domains from your email audience, especially if you are working to improve sender reputation and inbox placement.
Strategic exclusion for sender reputation improvement
The primary reason for identifying and excluding all Microsoft-owned inboxes is often tied to a strategy of repairing or rewarming sender reputation. When your sender reputation with a specific mailbox provider is low, reaching a 'volume zero' state for a period can allow the reputation to reset or stabilize. This means temporarily stopping all email traffic to those specific domains.
Microsoft SNDS (Sender Network Data Services) provides valuable insights into your sending reputation with Microsoft properties. If you're seeing persistent send volume there, even after removing common domains, it's a strong indicator that you haven't identified all relevant Microsoft-owned inboxes. The goal is to see a significant drop, or ideally, zero volume, during your rewarming period.
While this strategy can be effective, it requires diligence and a clear understanding of the domains involved. Misidentifying or missing certain domains can prolong the rewarming process or even hinder your efforts to improve deliverability. It is also important to consider if you need to exclude high risk email addresses from your lists.
Benefits of comprehensive exclusion
Improved sender reputation: A clean exclusion helps reset your standing with Microsoft's filters.
Reduced blocklisting risk: Minimizing sends to problematic addresses helps you avoid blacklists.
Accurate performance metrics: Better data if you're not sending to unwanted addresses.
Challenges of incomplete exclusion
Prolonged rewarming: Small volumes can keep your reputation suppressed.
Ongoing deliverability issues: Emails may continue to land in spam or be blocked.
Misleading SNDS data: Inaccurate reporting of your actual sending to Microsoft.
Understanding how email blacklists actually work is critical for a comprehensive email deliverability strategy. When working with Microsoft, especially when trying to mitigate deliverability issues, thorough domain exclusion is a foundational step. Remember that even small, unnoticed send volumes can impact your reputation and the effectiveness of your rewarming efforts.
Implementing a robust exclusion process
To systematically identify and exclude all Microsoft-owned inboxes, you'll need to combine known domain lists with active MX record lookups. Start by compiling all the domains you currently send to. For any domain that isn't immediately recognized as a major ISP (like Google, Yahoo, etc.), perform an MX record check. This process helps you pinpoint domains that might be lesser-known but are still routed through Microsoft's mail servers.
Maintain a dynamic exclusion list that you regularly update. Mailbox providers, including Microsoft, can change their infrastructure, and new domains might begin using their services. Regularly auditing your email list for these domains, alongside identifying disposable email domains, is crucial for ongoing list hygiene and deliverability success. Automating this process where possible can save significant time and reduce manual errors.
Important: Business domains vs. consumer domains
While many business domains use Microsoft 365, their impact on your SNDS reputation for consumer mail might be less direct. SNDS primarily focuses on the consumer-facing Hotmail, Outlook, and Live properties. However, if your goal is a complete halt to all traffic to any Microsoft-managed inbox for rewarming or compliance reasons, including these business domains is essential. You may also need to monitor for bot clicks and opens which can often originate from Microsoft/Outlook domains and skew your engagement metrics.
By diligently applying these identification and exclusion strategies, you can gain better control over your email sending practices. This granular control not only helps in reputation management but also ensures that your email campaigns are targeting the correct audience segment, leading to higher engagement and better overall deliverability. It's a proactive step in preventing your emails from landing in spam or being caught by a blocklist (or blacklist) in the future.
Microsoft 365 (formerly Office 365) for businesses.
Primarily affects B2B deliverability, less direct impact on consumer SNDS.
%.mail.outlook.com
Academic/Specialized (often for educational institutions).
Impacts deliverability to these specific niche segments.
Views from the trenches
Best practices
Regularly perform MX lookups on your full subscriber list to identify Microsoft-hosted domains, including less common ones and international variations.
Segment your audience by mailbox provider to apply specific sending strategies, like temporary exclusions for reputation rebuilding.
Use Microsoft SNDS to monitor your reputation and verify the effectiveness of your exclusion efforts.
Common pitfalls
Relying only on common Microsoft domains (e.g., hotmail.com, outlook.com) and missing country-specific or lesser-known variations.
Assuming B2B domains hosted on Microsoft 365 have no impact on consumer SNDS data, leading to incomplete exclusion.
Failing to monitor send volume in SNDS after implementing exclusions, which can indicate overlooked domains.
Expert tips
A comprehensive list of Microsoft-owned domains can be extensive due to international variations and hosted B2B accounts. A good starting point can be found on reputable deliverability resource websites, but dynamic MX lookups offer the most current data.
For B2B domains, while they might not show up in consumer-focused SNDS, performing MX lookups on those domains and excluding them can still be beneficial for overall deliverability and compliance.
The distinction between Microsoft's free webmail MX records (like olc.protection.outlook.com) and Office 365 hosted MX records (like mail.protection.outlook.com) remains a reliable indicator for segmentation.
Expert view
Expert from Email Geeks says: There are dozens of Microsoft-owned domains, especially many country extensions, that can contribute to your send volume and need to be identified beyond the common ones.
October 1, 2022 - Email Geeks
Expert view
Expert from Email Geeks says: I would recommend doing MX lookups on all domains you send to and identify those pointing to Microsoft's webmail MX records (e.g., olc.protection.outlook.com) or Office 365 hosted MX records (e.g., mail.protection.outlook.com).
October 1, 2022 - Email Geeks
Refining your email audience and deliverability
Identifying all Microsoft-owned inboxes is a crucial step for any sender aiming for high deliverability. It's not just about excluding the popular domains; it's about a deep dive into your entire audience to catch every variant and hosted domain. This diligence ensures that when you pause sending to Microsoft addresses, you truly are achieving zero or near-zero volume, allowing your sender reputation to improve effectively.
Remember that email deliverability is an ongoing process that requires continuous monitoring and adaptation. Tools like Microsoft SNDS are invaluable for providing feedback on your sending practices to their network. By combining this feedback with a rigorous process of identifying and managing your audience, you can proactively avoid deliverability issues and maintain a healthy sender reputation, reducing the likelihood of being placed on a blocklist (or blacklist).
Ultimately, a cleaner and more segmented email audience translates directly into better inbox placement and campaign performance. Don't underestimate the impact of those seemingly small volumes to lesser-known Microsoft domains. Every email sent to an unwanted or problematic address can detract from your overall deliverability, making this comprehensive exclusion strategy a vital part of your email marketing success.
Microsoft. Many email senders try to exclude Microsoft-owned inboxes from their audiences to manage sender reputation or execute specific rewarming strategies. The challenge often lies in identifying all of them, beyond the obvious ones like @hotmail.com or @outlook.com. Even after diligently removing these known domains, you might still observe send volume reported in Microsoft SNDS, indicating that other Microsoft-controlled inboxes are still present in your audience.
Office 365 (now Microsoft 365) typically point to mail.protection.outlook.com. There are also specific endpoints for academic or government clients, such as those pointing to mail.outlook.com. By taking a list of domains you send to and performing MX lookups, you can build a more accurate exclusion list.
Google, Yahoo, etc.), perform an MX record check. This process helps you pinpoint domains that might be lesser-known but are still routed through Microsoft's mail servers.
