How to resolve Microsoft deliverability issues during IP warm up after being blocked?
Michael Ko
Co-founder & CEO, Suped
Published 30 Jul 2025
Updated 16 Aug 2025
9 min read
Dealing with Microsoft (Outlook, Hotmail, MSN) deliverability issues during an IP warm-up can be incredibly frustrating. I've seen it happen countless times, where a perfectly planned warm-up hits a brick wall because Microsoft suddenly blocks all traffic from a new IP address. It feels like you're doing everything right, gradually increasing your volume, and then, bam, a complete rejection.
This usually happens when their automated systems detect unusual sending patterns, which can be typical for a new IP that hasn't built sufficient reputation yet. The bounce messages often point to a general network block (like the S3150 error), rather than a specific blacklist listing, which can be confusing because you might think it's related to a public blocklist like SORBS, even if Microsoft isn't actually using it.
The key is to understand Microsoft's unique approach to email filtering and how to navigate their support channels effectively. It's a game of persistence and demonstrating that you are a legitimate sender. Let's dive into how to tackle these issues head-on and get your emails back into inboxes.
Sometimes, it's not even a block, but rather an issue with the Microsoft 451 4.7.650 error during email IP warm-up which can also be a challenge.
Microsoft's email filtering logic
Microsoft's email filtering systems, particularly for Outlook, Hotmail, and MSN domains, are highly sophisticated and often rely on a sender's established reputation. When a new IP address begins sending, it starts with a neutral reputation. Any sudden increase in volume, especially if the content is deemed questionable or if there are spikes in complaints, can quickly trigger their spam filters, leading to blocks.
Unlike some smaller mailbox providers, Microsoft typically doesn't directly reference many public blocklists (or blacklists) like SORBS. If you receive a bounce message like S3150, which indicates a block due to "part of their network is on our block list," it's usually an internal block triggered by their reputation system. This means getting delisted from a third-party blocklist might not automatically resolve the issue with Microsoft. You can refer to Microsoft's warm-up process for email marketing for more details.
Their system prioritizes user engagement and adherence to best practices, including robust email authentication (SPF, DKIM, DMARC). A sudden influx of emails from a new IP during warm-up can be flagged as suspicious, even if your content is legitimate. This is a common hurdle, but one that can be overcome with a targeted strategy.
Understanding Microsoft's S3150 error
When you see the S3150 error, it indicates that Microsoft's system has flagged your IP due to perceived reputation issues within their network, not necessarily because of a public blocklist. It's a generic block that requires direct intervention through their support channels.
Immediate response: contacting Microsoft support
The most effective way to address a Microsoft block during IP warm-up is to open a support ticket with the Microsoft Outlook.com Postmaster team. You can do this through their Outlook.com Postmaster page. Be prepared for an initial automated response that might state they cannot mitigate the issue or refer you to their Sender Network Data Services (SNDS) program. This is a common first step and shouldn't deter you.
When you receive that first boilerplate reply, respond to it, clearly explaining your situation. State that you are warming up a new IP address, provide your sending IP, and describe your planned traffic volume over a few date milestones. Emphasize that your emails are legitimate and that you are following best practices. This persistent follow-up is crucial because it signals to their system (and potentially a human reviewer) that you are a serious sender trying to resolve a legitimate issue.
Often, after a few exchanges, you will receive a response indicating that the issue has been mitigated, and your emails should begin delivering within 24-48 hours. If the problem persists, escalate your request within the same ticket thread. Providing clear facts, evidence of your sending practices, and a consistent, professional approach tends to yield the best results. For specific Microsoft IP warming issues, direct communication is often the most effective path.
Example Microsoft S3150 bounce message
5.7.1 (delivery not authorized) Unfortunately, messages from [IP ADDRESS] 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
Proactive warm-up and prevention
While resolving a current block is important, preventing future issues is even more critical. A well-executed IP warm-up strategy is foundational. This involves starting with a very low volume of emails and gradually increasing it over several weeks. Your initial sends should target your most engaged subscribers to build a positive reputation quickly. Avoid sending large volumes of emails to inactive or unengaged users, especially early in the warm-up process.
Beyond volume control, robust email authentication is non-negotiable for Microsoft deliverability. Ensure your SPF, DKIM, and DMARC records are correctly configured. These protocols help prove that your emails are legitimate and prevent spoofing. Microsoft heavily relies on these signals to assess sender authenticity. You can read a simple guide to DMARC, SPF, and DKIM.
Maintaining a clean and engaged email list is also crucial. Regularly remove inactive subscribers and avoid sending to purchased or old lists. High bounce rates or spam complaints, even during warm-up, can quickly derail your efforts. Monitor your sender reputation closely and adjust your sending patterns if you notice any dips. For issues with low sending volume, you can check out fixing IP warming strategies.
Best practices
Gradual volume increase: Start with minimal volumes and slowly scale up over weeks.
Engaged recipients: Prioritize sending to your most active subscribers first.
Email authentication: Ensure SPF, DKIM, and DMARC are properly set up.
List hygiene: Regularly clean your lists to remove inactive or invalid addresses.
Content quality: Send relevant and valuable content to minimize complaints.
Common pitfalls
Sudden volume spikes: Sending too many emails too soon from a new IP.
Sending to unengaged lists: Targeting recipients who haven't interacted in a long time.
Poor authentication: Incorrect or missing SPF, DKIM, or DMARC records.
Ignoring bounce messages: Not analyzing and acting on error codes promptly.
Not monitoring reputation: Failing to track deliverability metrics and blocklist status.
Building long-term sender reputation
Beyond the immediate fix for a block, building and maintaining a strong sender reputation with Microsoft is an ongoing effort. Sign up for the Microsoft Sender Network Data Services (SNDS) program. SNDS provides data on your sending IP's reputation, complaint rates, and spam trap hits from Microsoft's perspective. It's an invaluable tool for understanding how your sending practices are perceived by them.
Regularly monitor your complaint rates. High complaint rates are a major red flag for Microsoft and can quickly lead to blocks or spam folder placement. Ensure your email content is relevant and your unsubscribe process is clear and easy. Implementing a clear unsubscribe option helps reduce complaints, as recipients are less likely to mark your emails as spam if they can easily opt out.
Finally, consistent sending practices are key. Avoid long periods of inactivity followed by sudden bursts of email. A steady, predictable sending pattern at appropriate volumes helps maintain a positive reputation. If you're encountering persistent IP reputation issues despite warm-ups and clean lists, review your content and engagement metrics more deeply.
Navigating IP blocks for new sending IPs
When migrating to a new ESP (Email Service Provider) or encountering unexpected blocks, it's essential to quickly diagnose the root cause. This often means going beyond simple blocklist checks and delving into bounce codes, sender reputation, and engagement metrics to understand the full picture.
Even for large providers like Salesforce Marketing Cloud, you can still experience Microsoft deliverability issues during IP warm-up. This is where patience and persistence with Microsoft's support team become vital. Their initial responses are often automated, but persistent engagement with detailed information can lead to manual review and resolution.
For specific block codes, such as the S3150 often encountered during IP warming, the solution typically involves a direct request for accommodation or delisting from Microsoft's internal systems. While public blacklists (or blocklists) like SORBS are less likely to directly impact Microsoft deliverability, it's still good practice to monitor your IPs for listings on major email blocklists and address any legitimate issues.
Ultimately, a combination of proactive warm-up strategies, diligent monitoring, and persistent communication with mailbox providers like Microsoft is essential to navigate the complexities of email deliverability, especially when establishing a new sending IP.
Views from the trenches
Best practices
Maintain strong authentication (SPF, DKIM, DMARC) for all sending domains.
Start IP warming with highly engaged subscribers to build positive reputation.
Segment your audience based on engagement to protect new IPs from low-engagement sends.
Monitor Microsoft's SNDS daily for any reputation issues or delivery alerts.
Always keep bounce logs handy to understand the exact error codes from Microsoft.
Common pitfalls
Sending large volumes too early in the warm-up process.
Ignoring generic bounce messages from Microsoft without persistent follow-up.
Assuming a public blacklist listing (like SORBS) is the sole reason for Microsoft blocks.
Failing to articulate your warm-up plan and sending patterns to Microsoft support.
Not regularly cleaning your email list, leading to higher spam complaints.
Expert tips
The initial automated responses from Microsoft support are rarely useful; always reply and ask for escalation, providing detailed context about your IP warming.
Microsoft's legal team often restricts the communication from their support, but humans are still reviewing requests, so persistence is key.
Include a professional title in your email signature when communicating with Microsoft support, as it can sometimes help your ticket get more attention.
Microsoft's blocks, like the S3150, are often internal reputation-based and not directly tied to public blocklists like SORBS.
Even if they claim to have mitigated the issue, always verify by testing your sends and be prepared to follow up if delivery problems persist.
Marketer view
Marketer from Email Geeks says they were two weeks into IP warm-up with Salesforce Marketing Cloud when Microsoft domains started completely rejecting their emails, thinking it was related to a SORBS listing.
2023-04-11 - Email Geeks
Expert view
Expert from Email Geeks says Microsoft likely does not reference SORBS directly and questioned the exact bounce being thrown, noting that major providers generally do not rely on SORBS for deliverability impact.
2023-04-11 - Email Geeks
Final thoughts on Microsoft deliverability
Getting blocked by Microsoft during IP warm-up is a common challenge, but it is certainly not insurmountable. The key is to remember that Microsoft's systems prioritize sender reputation and good sending practices. This means going beyond basic blocklist checks and engaging directly with their postmaster team.
My advice is to be proactive with your warm-up, meticulously set up your authentication records, and be prepared to be persistent with Microsoft support. They may not give you personalized responses, but a consistent, detailed, and polite follow-up can often lead to a resolution. By combining these strategies, you can successfully navigate Microsoft's filters and ensure your emails reach the inbox.
Microsoft isn't actually using it.
