What should I do if Microsoft blocks my IP address during IP warming?
Matthew Whittaker
Co-founder & CTO, Suped
Published 23 May 2025
Updated 17 Aug 2025
5 min read
Getting an IP address blocked by Microsoft during the critical IP warming phase can be a frustrating experience. You're diligently building your sender reputation, carefully increasing volume, and then suddenly, messages destined for Outlook or Hotmail addresses bounce back. It feels like hitting a wall when you're just getting started.
This setback is more common than you might think, and it doesn't necessarily mean your entire warming strategy is flawed. Mailbox providers, especially Microsoft, are highly sensitive to new sending patterns. They prioritize user safety and inbox quality, which sometimes leads to aggressive filtering of unfamiliar IP addresses, even if your intentions are legitimate. The key is to understand what's happening and respond effectively to get back on track.
Understanding the nature of the block
The very first step when Microsoft (or any mailbox provider) blocks your IP address is to pinpoint the exact reason. Your Email Service Provider (ESP) might give you a generic message, but you need to dig deeper. The actual bounce message contains critical information that will guide your remediation efforts.
Common Microsoft block codes include S3150 and S3140, indicating a network blocklist issue, or a 451 4.7.650 S775 error, which points to a temporary rate limit due to IP reputation. Knowing which specific code you're receiving is crucial for determining if it's a temporary setback or a more serious reputation problem. If you're using a platform like Marketing Cloud, you might need to query their data views to retrieve the precise SMTP rejection codes.
Example of a common Microsoft bounce messageplaintext
550 5.7.1 Unfortunately, messages from [X.X.X.X] 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.
Once you have the specific bounce code, it's easier to decide on the next course of action. Generic messages from your ESP are rarely enough to diagnose the root cause accurately, which is essential for effective troubleshooting. Often, the issue may be a temporary rate limit that resolves itself with a slight adjustment in sending volume.
Obtain precise bounce messages
Do not rely solely on generalized ESP bounce classifications. These often mask the true underlying issue. Request or extract the full SMTP bounce response from your logs.
Accurate bounce codes, like S3150 or 451 4.7.650, provide direct clues about Microsoft's reason for blocking your IP, allowing for targeted remediation.
Immediate steps and Microsoft engagement
After identifying the bounce message, you face a critical decision during IP warming: should you stop sending, continue sending but suppress Microsoft addresses, or continue sending as normal? Generally, if you're getting hard bounces or specific blocklist error codes, it's best to pause or significantly reduce sends to affected domains until you have a clear resolution plan.
A crucial proactive step, if you haven't already taken it, is to request pre-emptive accommodation from Microsoft. This involves notifying them that a new IP address will be sending mail and providing details about your sending volume and type. While it doesn't guarantee smooth sailing, it can certainly help to build trust from the outset. You can find more information about requesting pre-emptive accommodation on Spamresource.com.
Risk of continued sending
Worsening reputation: Continued sending to a blocklisted (or blacklisted) IP can further degrade your sender reputation with Microsoft, making future delisting harder.
No deliverability: If emails are being blocked, continuing to send to those addresses is pointless and wastes resources.
Increased spam complaints: For messages that do get through, recipients might mark them as spam, compounding the reputation damage.
Recommended approach
Temporarily pause or suppress: Halt sends to Microsoft-owned domains (Outlook, Hotmail, Live) until the block is lifted and you have a clear plan.
Contact Microsoft support: Use the Office 365 Anti-Spam IP Delist Portal to request delisting. Provide all necessary details, including the bounce message.
Provide a ramp-up plan: When communicating with Microsoft, include a revised, slower ramp-up plan specifically for their domains to demonstrate your commitment to good sending practices.
Once you've contacted Microsoft, they will evaluate your request. Be prepared to provide detailed information about your sending practices and the emails you're sending. It's a collaborative process to convince them that you are a legitimate sender. For additional information on their policies, the Microsoft Postmaster website is a valuable resource.
Building and maintaining a strong sender reputation
Beyond the immediate fix, it's essential to reassess your overall sender reputation and IP warming strategy to prevent future blockages. Microsoft is very sensitive to recipient engagement and spam complaints. If your list contains inactive or low-engagement addresses, it can negatively impact your sender score, leading to blocks.
Regularly cleaning your email list to remove unengaged or invalid addresses is crucial. Also, ensure your email authentication protocols—SPF, DKIM, and DMARC—are correctly configured. These measures help mailbox providers verify that your emails are legitimate and reduce the likelihood of your IP address being blocklisted (or blacklisted). You can check why Microsoft might be blocking your IP even after warming up.
Factor
Description
Action
Sender Score
Microsoft, like other providers, assesses your IP's sending reputation.
Remember that IP warming is a delicate process, and occasional blockages, especially by providers like Microsoft, are part of the journey. The goal is to build trust over time by demonstrating consistent, legitimate sending behavior. Patience and persistence are key to overcoming these challenges and achieving optimal deliverability.
Best practices to prevent future blocks
Segment your audience: Start warming with your most engaged subscribers who are likely to open and click.
Gradual volume increase: Adhere strictly to your warming schedule and adjust slowly if issues arise.
Monitor engagement metrics: Pay close attention to opens, clicks, and spam complaints, especially for Microsoft domains.
List hygiene: Regularly remove hard bounces and unengaged subscribers to maintain a clean list.
Views from the trenches
Best practices
Always obtain the exact SMTP bounce message before taking any action, as generic ESP classifications can be misleading and not actionable.
Proactively engage with Microsoft by submitting a pre-emptive accommodation request when warming new IP addresses.
Prepare a detailed, slower ramp-up plan specifically for Microsoft domains to share during any delisting or communication.
Common pitfalls
Continuing to send emails to Microsoft domains while your IP is blocked can severely damage your sender reputation.
Failing to clean your email lists of unengaged recipients, which can trigger spam filters and lead to blocks.
Not having a clear understanding of your bounce codes, leading to incorrect assumptions and ineffective remediation strategies.
Expert tips
Automate the process of pulling bounce data into data extensions for programmatic use, enabling faster analysis.
Recognize that Microsoft can be finicky during IP warming, and proactive communication can help manage expectations.
If an entire block of IPs is listed, your ESP will need to resolve it, as you may be caught up in a broader network issue.
Expert view
Expert from Email Geeks says Microsoft can be quite particular during even the most carefully planned IP warm-ups, even if you have proactively provided them with daily volume projections.
2023-12-15 - Email Geeks
Expert view
Expert from Email Geeks says ESP bounce classifications are often unreliable, and it is crucial to obtain the actual rejection message before determining the next steps.
2023-12-15 - Email Geeks
Navigating Microsoft IP blocks and beyond
Encountering a Microsoft IP block (or blacklist) during IP warming is a significant hurdle, but it is manageable with the right approach. The core of your response should always be rooted in understanding the precise bounce message and engaging proactively with Microsoft. This initial diagnosis dictates whether you're dealing with a temporary rate limit or a more persistent reputation issue.
By pausing or adjusting your sending to affected domains and presenting a clear remediation and re-warming plan, you demonstrate to Microsoft that you are a responsible sender committed to best practices. Remember that consistent monitoring of your email deliverability and proactive list management are long-term strategies that help maintain a healthy sender reputation and avoid future blocks, ensuring your messages consistently reach the inbox.
Microsoft, are part of the journey. The goal is to build trust over time by demonstrating consistent, legitimate sending behavior. Patience and persistence are key to overcoming these challenges and achieving optimal deliverability.
