Why are Microsoft email addresses bouncing and how do I fix it?
Michael Ko
Co-founder & CEO, Suped
Published 6 Aug 2025
Updated 16 Aug 2025
8 min read
Dealing with email bounces from Microsoft email addresses, including Hotmail, Outlook.com, and MSN, can be a particularly frustrating experience. Often, these issues appear suddenly, even when your overall email deliverability to other providers seems perfectly fine. This specific targeting by Microsoft's systems indicates there are unique factors at play that need to be understood.
My experience has shown that these bounces are rarely random. There's almost always a specific reason, often related to IP reputation, sender authentication, or content quality, even if standard blacklist checks show your domain and IP are clean. Microsoft employs its own internal blocklists (blacklists) and sophisticated filtering mechanisms that can be more sensitive than those of other Email Service Providers (ESPs). To effectively resolve these bounces, we need to delve deeper into the nature of the bounce messages themselves and then systematically address the underlying causes.
This guide will walk you through identifying the common reasons why Microsoft email addresses might be bouncing and provide actionable steps to fix these issues, helping you restore your email deliverability to their mailboxes.
Analyzing Microsoft bounce messages
The first and most critical step in troubleshooting any email bounce is to carefully examine the Non-Delivery Report (NDR), also known as the bounce message. This message is like a diagnostic report, providing specific error codes and descriptions that tell you exactly why Microsoft rejected your email. Without this information, you are essentially troubleshooting in the dark.
A common bounce message from Microsoft will look something like this:
Understanding the bounce code
Example Microsoft bounce message
smtp; 550 5.7.1 Unfortunately, messages from [54.240.46.27] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [AM6EUR05FT037.eop-eur05.prod.protection.outlook.com]
The key information here is the 550 5.7.1 error code and the phrase "part of their network is on our block list (S3150)." This explicitly tells you that the sending IP address (in this case, 54.240.46.27) is on a Microsoft internal blocklist. While public blacklist checkers might show your IP as clean, Microsoft maintains its own proprietary lists.
For a comprehensive understanding of various Microsoft non-delivery reports, you can consult their official documentation.
Another common scenario is when your IP address is part of a larger network range (a /24 or /22 block) that has been flagged due to the sending practices of other users within that same network segment. Even if your specific IP is well-behaved, the actions of a "noisy neighbor" can lead to blocklisting (blacklisting) of the entire range by Microsoft.
Common causes of Microsoft email bounces
Microsoft, like other major email providers, places significant emphasis on sender reputation. Your IP and domain reputation are crucial for email deliverability. While a low complaint rate (e.g., 0.001%) seems good, if you are sending large volumes, even a small percentage of negative signals can impact your reputation, especially if you have recently increased sending volume or acquired new customer lists.
Microsoft's systems are particularly sensitive to sudden changes in sending behavior, content, or volume. If you've recently onboarded a new customer or started sending different types of content, this can trigger their filters. This is especially true if the email list was not optimally maintained or if there are issues with its source. This can lead to emails not delivering to Microsoft inboxes.
Another key area is email authentication. Microsoft, along with other major providers like Gmail and Yahoo, has recently tightened its requirements for bulk senders. This means that robust SPF, DKIM, and DMARC implementation with proper alignment is no longer optional, but mandatory for consistent deliverability. In fact, if your SPF DNS record times out at Microsoft, your emails may fail to deliver.
Possible causes
IP reputation: Your sending IP is on a Microsoft internal blocklist (blacklist) or is part of a dirty IP range.
Sender reputation: High spam complaints or low engagement rates, even if overall numbers seem low.
Authentication issues: Incorrect or missing SPF, DKIM, or DMARC records, or alignment failures.
Content quality: Spammy keywords, broken links, or suspicious attachments.
Invalid recipients: Sending to non-existent or inactive Microsoft email addresses.
Potential impact
Hard bounces: Permanent delivery failures, leading to high bounce rates.
Spam folder placement: Emails delivered but routed directly to the junk folder, reducing visibility and engagement.
IP/domain throttling: Microsoft temporarily limits the number of emails you can send.
Reduced sender reputation: Long-term damage to your ability to reach Microsoft inboxes.
Understanding these potential causes is the first step towards formulating an effective solution. It's often a combination of factors rather than a single issue that leads to persistent bouncing.
Diagnosing and resolving issues
Once you've analyzed the bounce message, it's time to take specific actions to address the root cause. This often involves checking your sending infrastructure, authentication records, and overall sending practices.
For IP-related issues, especially if you're using a cloud-based email service provider (ESP) like Amazon SES, even if you have dedicated IPs, your reverse DNS (rDNS) might still point back to the ESP's generic domain. Custom rDNS can make your sending IP appear more legitimate to Microsoft. You'll need to work with your ESP to set this up, as it clarifies that these are your dedicated IPs and not part of a broader, potentially problematic shared pool.
Next, verify your SPF, DKIM, and DMARC records. Microsoft's requirements are stringent. Ensure your records are correctly published in your DNS and that both SPF and DKIM align with your DMARC policy. You can use an email deliverability tester to check these settings quickly.
Beyond technical configurations, list hygiene is paramount. Sending to invalid or infrequently engaged addresses signals poor list quality to Microsoft's filters. Regularly clean your lists to remove hard bounces and inactive users. Tools can help with this, but it's an ongoing process. You might also want to review why a valid email address hard bounced.
Finally, monitor your feedback loops and complaint rates via services like Microsoft SNDS. While gaining access to SNDS data can be challenging if you don't directly own the IP space (as with some ESPs), it's worth pursuing. Low complaint rates combined with high bounces can sometimes mean your emails are going straight to spam, which doesn't always generate a complaint report but still impacts deliverability.
Bounce codes and immediate fixes
Here's a quick reference table for common Microsoft bounce codes and what they typically mean, along with initial steps:
Bounce code
Meaning
Action to take
550 5.7.1
Sender IP or network is blocklisted by Microsoft due to spam or reputation issues.
Check SNDS (Smart Network Data Services). Request delisting. Investigate if your network neighbor is causing problems. Consider custom rDNS.
550 5.4.1
Recipient address does not exist or mailbox is full.
Verify the recipient email address. Remove invalid addresses from your list. Implement regular list cleaning.
550 5.2.1
Mailbox disabled or inactive. Similar to 5.4.1.
Remove the address from your active sending list to prevent further bounces.
550 5.7.350
Policy violation. Often related to new sender requirements.
Temporary deferral, often due to high volume or suspicious activity.
Reduce sending volume and gradually increase. Check content for spam triggers. Monitor sender reputation.
Persistent issues with Microsoft email addresses bouncing may also indicate deeper underlying problems with your overall email program. It's crucial to adopt a holistic approach to email deliverability, continuously monitoring your sender health and adapting to evolving requirements. Regularly checking your DMARC reports can also provide valuable insights into authentication failures that Microsoft may be experiencing.
Advanced troubleshooting and preventative measures
Proactive measures for Microsoft deliverability
Maintaining consistent email deliverability, especially to a major provider like Outlook.com, requires ongoing effort. Here are some key proactive steps:
List hygiene is continuous: Implement a robust email list cleaning process to regularly remove inactive or invalid addresses.
Monitor engagement: Actively track open rates, click-through rates, and, most importantly, complaint rates.
Content quality: Avoid spammy content, excessive images, or too many links. Personalize where possible.
Gradual volume increases: If you're using a new IP or domain, warm it up gradually before sending large volumes.
Proactive monitoring and adherence to best practices can significantly reduce your chances of experiencing unexpected bounces or being blocked by Outlook.
It is also worth noting that if you use a third-party ESP and they are facing issues with Microsoft's systems, they are usually in the best position to contact Microsoft's deliverability support team for mitigation. These teams often have established channels and processes for resolving blocklist (blacklist) issues or getting IPs delisted. If the issue persists, consider escalating it with your ESP to ensure they are actively working to resolve the problem on their end.
Remember, the email ecosystem is dynamic. What works today might need adjustments tomorrow. Staying informed about changes from major mailbox providers and regularly reviewing your email program's health is key to long-term deliverability success, especially when Microsoft email deliverability suddenly decreases.
Views from the trenches
Best practices
Always begin troubleshooting by thoroughly examining the full bounce message from Microsoft to pinpoint the exact error code and description.
Proactively monitor your IP and domain reputation using Microsoft's Smart Network Data Services (SNDS) for insights into your sending health.
Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned to meet Microsoft's strict authentication requirements.
Implement consistent list hygiene practices to regularly remove inactive or invalid email addresses, improving overall list quality.
Common pitfalls
Ignoring bounce messages and continuing to send to problematic addresses can further damage your sender reputation with Microsoft.
Assuming public blacklist checkers are comprehensive; Microsoft maintains its own internal blocklists that may not appear externally.
Neglecting proper email authentication, which can lead to emails being blocked or routed to the spam folder by Microsoft's systems.
Sending large volumes of email to newly acquired lists without proper warm-up, triggering Microsoft's spam filters.
Expert tips
If using dedicated IPs from an ESP, request custom reverse DNS (rDNS) to strengthen your sending identity and build trust with Microsoft.
For IP-related blocks, especially if you're on a shared network, ask your ESP to investigate whether 'noisy neighbors' are impacting your deliverability.
Even with low complaint rates, if bounces are high, it's possible emails are going directly to spam, which still requires content and reputation adjustments.
Maintain an open line of communication with your Email Service Provider (ESP) or IT team regarding Microsoft deliverability issues.
Expert view
Expert from Email Geeks says always examine the bounce messages first, as they provide exact details on why Microsoft is blocking your mail.
2022-10-20 - Email Geeks
Expert view
Expert from Email Geeks says if you are sending mail from Amazon EC2/SES, Microsoft often has specific issues with those IPs. Check if you are on dedicated IPs or a shared pool.
2022-10-20 - Email Geeks
Maintaining positive deliverability
Resolving email bounces from Microsoft properties requires a combination of technical diligence and adherence to best sending practices. By meticulously analyzing bounce messages, ensuring proper email authentication (SPF, DKIM, DMARC), maintaining a clean email list, and proactively monitoring your sender reputation, you can significantly improve your deliverability to Microsoft inboxes. Remember that consistent effort and adapting to evolving email standards are key to long-term success.
If you continue to face challenges, don't hesitate to consult with your ESP or a deliverability specialist. Sometimes, an external perspective or direct engagement with Microsoft's support channels (if accessible) can provide the breakthrough you need to get your emails consistently delivered. Keeping a close eye on your deliverability metrics is always the best defense against unexpected bounces and blocklists (blacklists).
Microsoft email addresses, including Hotmail, Outlook.com, and MSN, can be a particularly frustrating experience. Often, these issues appear suddenly, even when your overall email deliverability to other providers seems perfectly fine. This specific targeting by Microsoft's systems indicates there are unique factors at play that need to be understood.
Gmail and 
Yahoo, has recently tightened its requirements for bulk senders. This means that robust SPF, DKIM, and DMARC implementation with proper alignment is no longer optional, but mandatory for consistent deliverability. In fact, if your SPF DNS record times out at Microsoft, your emails may fail to deliver.
Outlook.com, requires ongoing effort. Here are some key proactive steps:
Microsoft properties requires a combination of technical diligence and adherence to best sending practices. By meticulously analyzing bounce messages, ensuring proper email authentication (SPF, DKIM, DMARC), maintaining a clean email list, and proactively monitoring your sender reputation, you can significantly improve your deliverability to Microsoft inboxes. Remember that consistent effort and adapting to evolving email standards are key to long-term success.
