Why am I suddenly seeing an increase in bounces from Microsoft domains?
Matthew Whittaker
Co-founder & CTO, Suped
Published 24 Jul 2025
Updated 16 Aug 2025
8 min read
Experiencing a sudden surge in bounce rates from Microsoft domains like Outlook and Hotmail can be alarming, especially when your sending practices haven't changed. It often indicates an underlying issue with your sender reputation, even if your existing metrics appear to be stable. Microsoft, like other major mailbox providers, employs sophisticated filtering systems that can be highly sensitive to subtle shifts in sending patterns or content, leading to abrupt rate limiting or rejections.
This sudden increase can feel like it came out of nowhere, leaving senders puzzled about the cause. The key is to understand Microsoft's specific bounce codes and their implications, then systematically investigate potential triggers ranging from unexpected volume changes to subtle reputation shifts. Let's explore what might be happening and how to address it.
Understanding Microsoft's bounce codes and reputation
When you see a bounce message from Microsoft domains, it often includes a specific error code. One common code, 4.7.650 The mail server [IP address] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com (S843), indicates that your sending IP has been rate-limited. This isn't necessarily a hard block or a sign of being on a public blacklist, but rather Microsoft's system temporarily throttling your outgoing mail due to perceived reputation issues.
Understanding the S843 error
The S843 error from Microsoft indicates that your IP address is being temporarily rate-limited. This is usually a soft bounce, meaning messages are deferred rather than permanently rejected. While it might not immediately signify a widespread blocklist (blacklist) listing, it's a clear signal that Microsoft's systems have flagged your sending behavior.
Microsoft's Postmaster site, postmaster.live.com, is your primary resource for understanding your reputation with them. Even if SNDS (Smart Network Data Services) shows your IP as green with low complaints, a sudden rate limit suggests something has recently triggered their filters. This could be due to changes in their filtering algorithms or even minor shifts in your sending patterns that previously went unnoticed.
For more information on the S3150 and S3140 errors that also indicate a block by Microsoft, you can refer to this article on delisting.
IP reputation is a critical factor for email deliverability at Microsoft. It's not just about being on a public blacklist, but also about how individual ISPs, like Microsoft, perceive your sending behavior. Factors contributing to a poor IP reputation can include a sudden increase in volume, a spike in spam complaints (even low ones), or hitting spam traps.
Even if your SenderScore remains high, specific internal metrics at Microsoft could trigger rate limiting. Transactional emails, while generally having better engagement, are not immune to these issues, especially if the volume or content suddenly changes without proper warning to the receiving system. Microsoft is known to be very sensitive to sudden volume changes, even small ones.
Common causes for sudden spikes
Several factors can contribute to a sudden increase in bounces from Microsoft domains, even if your sending practices seem consistent. It's crucial to consider less obvious triggers that might be impacting your sender reputation.
Sudden volume changes: Even if you haven't drastically increased your sending, any deviation from your established daily volume, particularly if it's an unannounced spike, can trigger Microsoft's rate-limiting algorithms. This is especially true if you are sending transactional emails where volume should be relatively consistent.
Content issues: A change in your email content, even subtle, could inadvertently trigger spam filters. This includes new keywords, links, or attachments that are perceived as suspicious. High complaint rates (even low numbers for transactional emails) can lead to blocks.Feedback loop reports from Microsoft can confirm this.
Authentication issues: Misconfigurations or recent changes to your SPF, DKIM, or DMARC records can lead to emails being flagged. Even if your records were correct, Microsoft might have updated its parsing or enforcement rules. For example, Outlook's new mystery rejection has shown authentication issues affecting bounces.
Compromised accounts or systems: If one of your email accounts or a system connected to your sending infrastructure is compromised, it could be sending spam without your knowledge, severely damaging your IP reputation. This often leads to a sudden increase in bounces.
List hygiene issues: While less likely for transactional emails, if your recipient list has suddenly accumulated a high number of invalid or inactive addresses, it can lead to increased hard bounces (or soft bounces for rate limits) from Microsoft.
Microsoft's systems are constantly evolving, and sometimes a change in their internal algorithms or filtering criteria can lead to a sudden shift in deliverability for senders who were previously performing well. This is particularly true for domains like Outlook and Hotmail. For example, some users reported increased bounces from Microsoft due to a new quota bar that was gradually rolled out.
Even for transactional emails from reputable senders like banks or local governments, these changes can have an immediate impact. It's often not a direct blacklist entry, but rather a proactive measure by Microsoft to protect its users from potential spam or abuse, even if your sending is legitimate.
Diagnosing the problem
When facing a sudden increase in Microsoft bounces, systematic diagnosis is essential. Your first step should always be to review the bounce messages themselves, as they often contain valuable clues like the S843 error code mentioned earlier. This code points directly to an IP reputation issue and rate limiting.
Example Microsoft Bounce Message
4.7.650 The mail server [161.47.110.154] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com (S843)
Next, consult Microsoft's SNDS (Smart Network Data Services) for your sending IPs. While SNDS might show your IPs as green with low complaint rates, it's still the most authoritative source for your reputation with Microsoft. If this data looks normal, it reinforces the idea that an internal algorithm change or a very subtle shift in your sending patterns might be the culprit.
Check your own email metrics closely. Are your spam complaint rates truly at zero, or has there been a tiny, uncharacteristic uptick? Even a small increase in complaints for transactional emails can be a red flag. Also, review your mail logs for any signs of unauthorized sending or a compromised system. This is especially important if the bounce increase was sudden and affected multiple unrelated sending IPs.
Finally, ensure your email authentication (SPF, DKIM, DMARC) is correctly configured and aligned. A small misconfiguration or even an SPF DNS timeout can cause issues specifically with Microsoft. For persistent problems, consider submitting a Microsoft mitigation request through their support channels.
Remediation strategies
Once you've diagnosed the likely cause of your increased Microsoft bounces, you can implement targeted remediation strategies. The exact steps will depend on your diagnosis, but generally, they fall into proactive and reactive categories.
Proactive measures
Maintain consistent volume: Avoid sudden, unannounced spikes in sending volume to Microsoft domains. If you anticipate a large send, gradually increase your volume (warm-up) over several days.
Clean your list: Regularly remove inactive or bouncing addresses to maintain a healthy list, even for transactional sends. This reduces the risk of hitting spam traps.
Monitor feedback loops: Pay close attention to Microsoft's Feedback Loop (FBL) reports for any unusual complaint activity. A high complaint rate is a major red flag for deliverability issues, indicating that your emails might be seen as spam.
Reactive fixes
Submit a mitigation request: If you've ruled out other issues, formally request mitigation from Microsoft. Be persistent and provide all relevant details, including bounce messages, IP addresses, and the steps you've taken to investigate. This can help resolve sudden deliverability decreases.
Address content triggers: If content is suspected, revise your email templates to remove any potentially spammy phrases or problematic links. Test new content incrementally.
Investigate compromises: If you suspect a compromised account or system, immediately secure it, change passwords, and scan for malware. Any malicious sending must be stopped to recover your reputation.
Review Outlook's new sender requirements: Outlook (Microsoft) has new requirements for senders, particularly for bulk senders. Ensure your practices align with these updated guidelines, as non-compliance can lead to increased bounces and spam classifications. You might also see issues with Barracuda Networks.
Addressing these issues promptly can help improve your domain and IP reputation with Microsoft, reducing the bounce rate. Remember that recovery often takes time and consistent good sending practices. Monitoring your metrics using tools like Microsoft SNDS and your own analytics is vital for tracking progress. Also, check for any mentions of your IP or domain on email blocklists (blacklists) to ensure you haven't been listed, which can also contribute to delivery failures.
While you might not find a single, obvious reason for the sudden increase, a combination of factors, or even a minor change in Microsoft's filtering, can lead to rate limiting. Persistent follow-up with Microsoft support if you submit a mitigation request is also often necessary.
Views from the trenches
Best practices
Maintain consistent sending volumes to Microsoft domains, avoiding sudden spikes.
Continuously monitor your sender reputation metrics through Microsoft SNDS.
Ensure all email authentication (SPF, DKIM, DMARC) records are valid.
Proactively clean your email lists to remove inactive or invalid addresses.
Common pitfalls
Ignoring subtle changes in Microsoft's filtering algorithms or new quota bars.
Failing to submit mitigation requests or follow up persistently with Microsoft support.
Overlooking small, uncharacteristic upticks in spam complaints or FBL reports.
Not checking for compromised accounts or systems that might be sending spam.
Expert tips
If SNDS data looks good, consider that Microsoft might have changed its algorithms.
Always review bounce messages for specific error codes and their guidance.
For transactional emails, even low complaint rates can be a red flag; investigate any rise.
Be prepared to follow up many times when submitting remediation requests to Microsoft.
Expert view
Expert from Email Geeks says checking for a blocklist (blacklist) entry is the first step, though IPs often aren't listed on public ones for temporary rate limits. Microsoft SNDS should be the primary check.
2023-10-15 - Email Geeks
Marketer view
Marketer from Email Geeks says Microsoft is highly sensitive to volume changes. Even if sending volume hasn't increased drastically, any deviation from established patterns can trigger rate limiting.
2023-10-15 - Email Geeks
Conclusion
A sudden increase in bounces from Microsoft domains can be a complex issue, often stemming from subtle shifts in reputation, content, or Microsoft's evolving filtering criteria. While public blocklists (blacklists) may not always be the culprit, Microsoft's internal rate limiting (like the S843 error) is a strong indicator that your IP reputation needs attention.
By diligently monitoring your metrics, checking authentication records, and proactively engaging with Microsoft's support channels when necessary, you can effectively diagnose and remediate these issues, ensuring your transactional and marketing emails consistently reach the inbox.
Microsoft indicates that your IP address is being temporarily rate-limited. This is usually a soft bounce, meaning messages are deferred rather than permanently rejected. While it might not immediately signify a widespread blocklist (blacklist) listing, it's a clear signal that Microsoft's systems have flagged your sending behavior.
Microsoft's SNDS (Smart Network Data Services) for your sending IPs. While SNDS might show your IPs as green with low complaint rates, it's still the most authoritative source for your reputation with Microsoft. If this data looks normal, it reinforces the idea that an internal algorithm change or a very subtle shift in your sending patterns might be the culprit.
