How to detect and resolve Microsoft IP blocks for email deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 2 Jul 2025
Updated 18 Aug 2025
7 min read
Experiencing email delivery issues to Microsoft domains can be incredibly frustrating. One moment your campaigns are flowing, the next your IP address is on a Microsoft blocklist, and your emails are bouncing back. This isn't just a minor hiccup, it can severely impact your communication, sales, and overall business operations. Understanding why these blocks occur and, more importantly, how to detect and resolve them, is essential for maintaining strong email deliverability.
Microsoft, with its various services like Outlook.com and Microsoft 365, employs robust anti-spam measures to protect its users. While effective, these systems can sometimes flag legitimate senders due to poor sending practices, shared IP reputation issues, or sudden changes in email volume. The key is to act swiftly when a block occurs, diagnose the root cause, and follow the correct procedures for delisting and prevention.
Detecting Microsoft IP blocks
Understanding Microsoft bounce codes
When Microsoft blocks your IP, you'll typically receive a Non-Delivery Report (NDR) with a specific error code. One common bounce message indicates that a portion of the network is on a blocklist (or blacklist). These codes provide vital clues to the nature of the block.
Example Microsoft bounce message
5.7.1 (delivery not authorized) Unfortunately, messages from [128.245.242.12] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140).
The S3140 and S3150 error codes, for instance, often indicate a network-level blocklist (or blacklist) rather than just an issue with a single IP. This can be particularly challenging if you are sending from a shared IP address, as the actions of other senders on that same IP or network range can impact your deliverability, even if your own sending practices are sound. It is crucial to examine the precise bounce message for the specific error code and accompanying text.
To preemptively detect if your IP is blocklisted (or blacklisted) by Microsoft, regularly checking your email sending reputation is essential. While there isn't a direct pre-send tool that forecasts a Microsoft block, continuous monitoring and proactive measures can significantly reduce the risk. This involves staying vigilant about your sending metrics and utilizing available tools to assess your IP status, which we'll cover next. You can also monitor your IP against other common blocklists, a process that can be handled with blocklist monitoring.
For situations where your dedicated IP is blocked, the issue might stem from specific sending behaviors tied to your operations, rather than shared network activity. In these cases, a thorough review of your email practices, combined with direct engagement with Microsoft's support and delisting processes, becomes even more critical for a swift resolution.
Leveraging Microsoft tools for detection and delisting
Microsoft provides specific tools to help senders manage their reputation and request delisting. The two primary resources are Microsoft Smart Network Data Services (SNDS) and the Office 365 IP Delist Portal. Registering your sending IPs in SNDS gives you access to data on your email-sending reputation, including spam complaint rates and spam trap hits. This data is critical for understanding the health of your sending IP and diagnosing issues that could lead to a blocklist (or blacklist).
The Delist Portal is your direct avenue for requesting the removal of a blocklisted IP. When you encounter a block, this portal is where you'll submit your IP address for review. Microsoft's system will then evaluate your request, and if deemed eligible, your IP can be delisted. However, delisting is often a temporary fix if the underlying issues aren't addressed. It's a key part of resolving a block, but not the only step.
It's important to remember that these tools are for Microsoft-specific blocks. While they are invaluable for resolving issues with Microsoft domains, they won't tell you if you're on other independent blocklists (or blacklists). For a comprehensive view of your blocklist status across multiple providers, a dedicated blocklist checker is recommended.
Common causes of Microsoft blocks
Microsoft IP blocks rarely occur without a reason. Typically, they are a response to behaviors that Microsoft's systems identify as potentially spammy or harmful. Common culprits include high spam complaint rates, sending to invalid or non-existent email addresses (which can lead to spam trap hits), and inconsistent sending volumes.
Lack of proper email authentication, such as SPF, DKIM, and DMARC, is another significant factor. These protocols help recipients verify that emails are legitimate and truly originate from your domain. Without them, your emails are more likely to be viewed with suspicion and routed to spam folders or rejected outright.
Common issues
High complaint rates: Recipients marking your emails as spam.
Inconsistent sending patterns: Sudden spikes or drops in volume without proper IP warming.
Poor email list hygiene: Sending to unengaged or old contacts.
Consequences
IP blocklisting (or blacklisting): Your IP is added to a blocklist.
Emails blocked/rejected: Messages bounce or don't reach inboxes.
Reduced deliverability: Impact on campaign performance.
Damage to sender reputation: Long-term issues with inbox placement.
Strategies for resolution and prevention
Resolving a Microsoft IP block (or blacklist) involves a combination of immediate action and long-term strategy. First, use the Office 365 IP Delist Portal to submit a delisting request. Be honest and thorough in your submission, and ensure you have addressed any issues that may have led to the block. While it's often a quick process, sometimes it takes persistence.
Beyond delisting, improving your sender reputation is paramount. This includes actively managing your email list to remove inactive or invalid addresses, reducing spam complaints by sending relevant and expected content, and consistently authenticating your emails with SPF, DKIM, and DMARC records. Microsoft heavily relies on these authentication standards to determine email legitimacy.
Consider whether a dedicated IP address is a better fit for your sending volume and practices. While shared IPs can be cost-effective, they expose you to the risk of other senders' poor practices affecting your reputation. A dedicated IP gives you full control over its reputation, but it also requires careful warming up and consistent monitoring. For detailed steps on managing Microsoft blocks, you can find more guidance on resolving Microsoft IP blocks.
Continuous vigilance is key to preventing future blocks. Regularly monitor your sender reputation using SNDS and other blocklist (or blacklist) checkers. Implement strict list hygiene practices, ensure your content is engaging and relevant, and respond promptly to any deliverability warnings. Proactive management of your email program is the best defense against recurrent IP blocks.
Views from the trenches
Best practices
Maintain strict list hygiene, removing invalid or unengaged subscribers regularly.
Always include clear unsubscribe links and honor opt-out requests promptly.
Segment your audience and send relevant, personalized content to avoid spam complaints.
Implement and correctly configure SPF, DKIM, and DMARC for all sending domains.
Monitor your Microsoft SNDS data daily to catch reputation issues early.
Warm up new IP addresses gradually to build a positive sending history.
Common pitfalls
Ignoring bounce messages and NDRs, missing crucial block notifications.
Sending to purchased or old lists that may contain spam traps.
Relying solely on shared IPs without understanding their historical reputation.
Failing to authenticate emails properly with SPF, DKIM, or DMARC.
Sending inconsistent volumes of email, leading to sudden reputation drops.
Assuming delisting is a permanent fix without addressing root causes.
Expert tips
Use a multi-faceted approach to monitor deliverability, not just one tool.
Focus on recipient engagement metrics as a primary indicator of sending health.
Build strong relationships with your email service provider to understand their IP management practices.
Develop a clear internal protocol for responding to and resolving IP block incidents.
Invest in tools that provide deep insights into your email stream performance.
Periodically review your email content for anything that might trigger spam filters.
Marketer view
Marketer from Email Geeks says they often see Microsoft lift a block without providing a clear explanation for the initial block.
2024-05-15 - Email Geeks
Marketer view
Marketer from Email Geeks indicates that it's challenging to pinpoint the exact problematic IP within an entire network range if you don't have oversight of the whole network.
2024-06-01 - Email Geeks
Maintaining a healthy sender reputation
Microsoft IP blocks, while disruptive, are often resolvable with a structured approach. By understanding the common causes, effectively using Microsoft's own tools like SNDS and the Delist Portal, and committing to best practices in email list management and authentication, senders can significantly reduce their risk of being blocklisted (or blacklisted). Staying proactive and vigilant in monitoring your email program is the most reliable way to ensure your messages consistently reach recipients' inboxes.
Microsoft 365, employs robust anti-spam measures to protect its users. While effective, these systems can sometimes flag legitimate senders due to poor sending practices, shared IP reputation issues, or sudden changes in email volume. The key is to act swiftly when a block occurs, diagnose the root cause, and follow the correct procedures for delisting and prevention.
Microsoft Smart Network Data Services (SNDS) and the Office 365 IP Delist Portal. Registering your sending IPs in SNDS gives you access to data on your email-sending reputation, including spam complaint rates and spam trap hits. This data is critical for understanding the health of your sending IP and diagnosing issues that could lead to a blocklist (or blacklist).
