Why is Microsoft blocking my IP address even after warming up?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 Aug 2025
Updated 16 Aug 2025
6 min read
It is a frustrating scenario for any email sender: you have diligently warmed up your IP address, carefully increasing your sending volume to build a good reputation, yet your emails are still being blocked by Microsoft. This common problem can make it feel like your efforts are wasted, but understanding why it happens is the first step toward a solution.
The truth is, Microsoft's email filtering systems, including those used by Outlook and Hotmail, are incredibly sophisticated. They look beyond just sending volume during warming and delve deep into subscriber engagement, list quality, and technical configurations. If you are facing persistent blocks, it indicates that there is a deeper underlying issue at play.
Warming up an IP address primarily establishes sending volume patterns and helps build initial trust, but it does not magically override poor sending practices or negative recipient feedback. Persistent blocks mean Microsoft does not believe your recipients truly want your mail, regardless of your warming efforts.
Understanding Microsoft's strict filtering
Microsoft, with its vast user base, maintains extremely stringent anti-spam and anti-abuse policies to protect its users. Their filtering algorithms are designed to detect even subtle indicators of unwanted mail, which go beyond simple volume metrics. While IP warming helps establish a baseline, it is not a complete solution for poor sender reputation.
Factors such as user complaints, low engagement, high bounce rates, and spam trap hits weigh heavily on their decision to block an IP address or place it on a blocklist (or blacklist). These metrics directly reflect whether recipients find your emails desirable and relevant. A high number of complaints, even from a small send, can quickly trigger a block, negating any warming progress.
It is critical to remember that Microsoft's systems prioritize user experience. If users are marking your emails as spam, deleting them without opening, or simply not engaging, Microsoft views this as a strong signal that the mail is unwanted. This can lead to your IP being repeatedly blocked, even after attempts to delist it or restart warming.
Beyond warmup: The role of list quality and engagement
If your IP is getting blocked despite warming, the primary culprit is often the quality of your recipient list and your opt-in practices. Sending to unengaged or questionable contacts will quickly lead to negative feedback, regardless of how slowly you ramp up your volume.
Even if subscribers have given single opt-in consent, it may not be enough for Microsoft's stringent filters if they are not actively engaging with your content. Old, purchased, or scraped lists are notorious for containing spam traps and disengaged users, leading to high complaint rates and immediate blocklisting (or blacklisting).
Good practices
Double opt-in: Confirms subscriber intent and significantly reduces spam complaints.
Engaged segments: Start warming with your most active and recent subscribers.
List cleaning: Regularly remove inactive users, bounces, and complainers.
Common pitfalls
Single opt-in only: Can lead to accidental sign-ups and higher complaint rates.
Unengaged users: Sending to dormant subscribers often triggers spam filters.
Poor list hygiene: Accumulating invalid addresses or spam traps.
A block (or blacklist) by Microsoft often signals that the recipients simply do not want the mail, as indicated by a variety of user actions. This is why you cannot simply "technical" your way out of a deliverability problem if the core issue is unwanted mail.
Technical considerations and hidden culprits
While recipient behavior is paramount, technical misconfigurations can also contribute to blocks. Incorrect or missing email authentication records (SPF, DKIM, DMARC) can raise red flags with Microsoft's servers, even if your content and list are pristine. These records help Microsoft verify that your emails are legitimate and prevent spoofing.
Common bounce codes like S3150 and S3140 from Microsoft often indicate an IP block due to suspicious activity or poor sending practices. To understand the exact reason for a block or throttling, you absolutely need to obtain detailed bounce logs from your email service provider (ESP). These logs provide invaluable insights into Microsoft's specific rejection messages.
Example Microsoft SMTP bounce error indicating an IP blocktext
550 5.7.1 Service unavailable, Client host [IP Address] blocked using Spamhaus. To request removal from this list see http://go.microsoft.com/fwlink/?LinkId=526651. (S3150)
Additionally, ensure your sender infrastructure is not holding open connections for unusually long periods or exhibiting other technically problematic behaviors. While less common with reputable ESPs, self-hosted sending systems or custom integrations might encounter such issues, leading to rate limiting or blocks.
Remedial actions and prevention
If your IP is blocked, the first step is to use the Office 365 Anti-Spam IP Delist Portal to request delisting. However, a successful delisting is only a temporary fix if the underlying issues are not addressed. You must demonstrate to Microsoft that you are a sender of wanted mail.
Re-evaluate your list acquisition methods and strengthen your consent process, ideally moving to double opt-in. Implement a robust list hygiene strategy, regularly removing unengaged subscribers and invalid email addresses to reduce hard bounces and spam trap hits. You can also actively monitor your IP on blocklists to catch issues early.
Key steps for long-term success
List quality: Focus on sending to genuinely engaged and opted-in subscribers.
Engagement monitoring: Continuously track opens, clicks, and complaints to identify disengaged segments.
Content relevance: Ensure your email content matches subscriber expectations and provides value.
By focusing on a holistic approach that includes list quality, user engagement, and technical hygiene, you can overcome persistent blocks and achieve better deliverability with Microsoft email accounts. This proactive strategy ensures your messages consistently reach the inbox, even with Microsoft's strict filtering.
Views from the trenches
Best practices
Always prioritize sending to highly engaged, opted-in users during and after warming.
Implement a double opt-in process to confirm subscriber consent and reduce spam complaints.
Regularly clean your email lists by removing inactive or unengaged subscribers.
Common pitfalls
Sending to unengaged or old lists, even after initial IP warming.
Ignoring bounce messages and not analyzing them for block reasons (e.g., S3150/S3140).
Not using feedback loops to identify and remove users who mark emails as spam.
Expert tips
Focus on recipient behavior and content relevance, as these heavily influence Microsoft's filtering.
Ensure your DNS records, including SPF, DKIM, and DMARC, are correctly configured.
Understand that IP blocks by Microsoft often indicate a deeper problem with list hygiene or subscriber expectations.
Expert view
Expert from Email Geeks says that accessing logs is crucial for understanding why an IP address is being blocked.
2021-04-07 - Email Geeks
Expert view
Expert from Email Geeks says that it is not possible to resolve an IP block for sending unopted-in email through standard warming practices, as Microsoft will identify it as spam.
2021-04-07 - Email Geeks
Achieving long-term deliverability
Microsoft's consistent blocking of your IP address, even after a thorough warming process, highlights that email deliverability is a multi-faceted challenge. It is not merely about sending volume, but fundamentally about the quality of your recipient list and the relevance of your content.
To ensure long-term inbox placement with Microsoft, prioritize maintaining a healthy list of engaged subscribers, implement strong authentication, and continuously monitor your sending reputation and bounce logs. Addressing these core issues will pave the way for successful email delivery.
Microsoft, with its vast user base, maintains extremely stringent anti-spam and anti-abuse policies to protect its users. Their filtering algorithms are designed to detect even subtle indicators of unwanted mail, which go beyond simple volume metrics. While IP warming helps establish a baseline, it is not a complete solution for poor sender reputation.
