Why is my IP address blocked by Hotmail and how do I resolve it?
Michael Ko
Co-founder & CEO, Suped
Published 14 Jun 2025
Updated 19 Aug 2025
7 min read
Finding your IP address blocked by Hotmail, now part of Microsoft Outlook, can be a frustrating experience. It means your emails aren't reaching their intended recipients, leading to communication breakdowns and potential business impact. This type of block, often referred to as a blacklist or blocklist, is a common deliverability issue for senders.
When Microsoft blocks an IP address, it’s usually because their systems have detected suspicious activity or poor sending practices originating from that IP. This isn't just about spam, it's about maintaining a clean and trustworthy email ecosystem for their users. As a sender, understanding the root cause is the first step toward resolution.
I will walk you through the reasons why your IP might be blocked by Hotmail (Outlook) and provide practical steps to get your emails flowing again. We'll cover diagnosis, the delisting process, and how to implement best practices to prevent future disruptions.
Why Hotmail (Microsoft Outlook) blocks your IP address
Hotmail (Microsoft Outlook) employs sophisticated filtering systems to protect its users from unwanted email. When your IP address is blocked, it's usually a signal that your sending reputation has declined to a point where Microsoft deems your mail untrustworthy. This can happen for several reasons, often related to your sending behavior and list quality.
One primary reason is a high volume of user complaints. If Hotmail users consistently mark your emails as spam, it quickly signals to Microsoft that your mail is undesired, leading to a blocklist (or blacklist) placement. Even a small number of complaints relative to your total sending volume can trigger these blocks, as Hotmail prioritizes user experience above all else. This can also lead to issues like Hotmail rate limiting your transactional emails.
Another common cause is sending to invalid or old email addresses, which can result in high bounce rates or hits on spam traps. Spam traps are email addresses used by ISPs to identify spammers. Sending to them is a clear indication of poor list hygiene. Furthermore, security issues like a compromised server or open relay can cause your IP to be blocklisted if it's used to send spam or malicious content without your knowledge.
Good sending practices
Permission-based lists: Only send to recipients who have explicitly opted in to receive your emails.
Engaged recipients: Focus on sending to an active audience that interacts with your content.
Proper authentication: Implement SPF, DKIM, and DMARC to verify your sending identity.
Bad sending practices
Purchased lists: Sending to lists obtained without direct consent.
High complaint rates: Many recipients marking your emails as spam.
If you suspect your IP address is blocked by Hotmail (Outlook), the first sign will usually be bounce messages. These messages often contain specific error codes, such as 550 5.7.1 or S3150, indicating that the email was rejected due to a blocked sending IP. Microsoft provides a dedicated portal for senders to check their IP status.
You can check if your IP is on their blocklist using the Office 365 Anti-Spam IP Delist Portal. Simply enter your IP address to see its current status and, if blocked, initiate a delisting request. Keep in mind that this portal is specifically for Microsoft's internal blocklists, which are the primary concern for Hotmail deliverability.
Understanding Bounce Codes
Bounce messages are crucial for diagnosing deliverability issues. They provide error codes and descriptions that tell you why an email couldn't be delivered. For Microsoft, common bounce codes related to IP blocks include:
550 5.7.1: This general error often indicates that the sender's IP address is on a blocklist (or blacklist) or has a poor reputation.
S3150/S3140: These specific Microsoft errors denote that your sending IP has been blocked due to suspicious activity, typically linked to spam complaints or high volume of unwanted mail. For further details, you can consult Microsoft's Sender Support FAQ.
Beyond Microsoft's own tools, general blocklist checkers can also provide insight, though Hotmail might use additional internal factors. However, the most reliable source for a Hotmail IP block diagnosis is always Microsoft directly. You can find out more about why your IP address might be blacklisted and how to fix it.
Steps to resolve a Hotmail IP block
Once you've confirmed your IP is blocked by Hotmail (Outlook), the resolution process begins with addressing the underlying issues that led to the block. Simply requesting a delisting without fixing the problem will likely result in a re-listing.
Your first step should be to pause all non-essential sending to Hotmail recipients from the blocked IP. This allows the IP's reputation to cool down. While this is happening, thoroughly review your sending practices. Are you using a double opt-in process? Are you regularly cleaning your lists? Check for any signs of compromise on your sending infrastructure. For more details on this, you can check our guide on why your emails to Microsoft domains are blocked.
Delisting process
After addressing the root causes, use the Office 365 Anti-Spam IP Delist Portal to request removal from the blocklist (or blacklist). The process is generally straightforward. Fill out the required information accurately. It's important to be honest and provide as much detail as possible about the steps you've taken to remediate the issue.
Microsoft's response can vary. Sometimes, delisting is immediate, especially if it was a temporary block or a minor issue. Other times, it might take longer, or your request might initially be denied if they don't see sufficient improvement in your sending behavior. Be prepared to wait and potentially re-submit your request after further improvements.
Once delisted, you'll need to slowly reintroduce your email volume, a process known as IP warming. This helps rebuild your IP's reputation with Hotmail (Outlook) and other ISPs gradually, proving that your mail is wanted and legitimate. For more complex issues or if you find yourself frequently blocked, considering a dedicated IP for your email sending can offer more control over your reputation, though it also comes with the responsibility of managing that reputation carefully.
Preventing future blocks
Preventing future IP blocks requires a proactive approach to email deliverability. It's not enough to get off a blocklist, you need to implement long-term strategies to maintain a positive sending reputation with Hotmail and other major mailbox providers.
Regular list hygiene is paramount. This means regularly removing unengaged subscribers, hard bounces, and any known spam traps from your mailing lists. Implement a double opt-in process to ensure that all new subscribers genuinely want to receive your emails. This reduces the likelihood of spam complaints, which are a major trigger for IP blocks.
Beyond list quality, ensure your email authentication protocols are correctly configured. This includes SPF, DKIM, and DMARC. These records help mailbox providers verify that your emails are legitimate and sent by an authorized sender. Monitoring your blocklist status and engagement metrics, such as open and click rates, is also essential for maintaining a healthy sending reputation.
Views from the trenches
Best practices
Regularly monitor your IP and domain reputation using tools like Microsoft's SNDS and Postmaster Tools.
Implement a strict double opt-in process for all new subscribers to ensure explicit consent.
Segment your audience and send relevant content to improve engagement and reduce spam complaints.
Common pitfalls
Sending to outdated or purchased email lists without prior consent.
Ignoring bounce messages and continuing to send to invalid addresses.
Failing to monitor feedback loops and address user complaints promptly.
Expert tips
Actively participate in feedback loops to quickly identify and remove users who mark your emails as spam.
Segment your list based on engagement and send less frequently to less engaged segments.
If your IP is repeatedly blocked, consider investing in a dedicated IP address to better control your sending reputation.
Expert view
Expert from Email Geeks says that if the IP is blocked, the content's green status (indicating no content problem) is irrelevant because Hotmail measures IP and content independently.
2020-04-21 - Email Geeks
Expert view
Expert from Email Geeks says the real reason for a block is that Hotmail believes the IPs are sending too much mail that their users do not want.
2020-04-21 - Email Geeks
Maintaining email deliverability
Dealing with an IP address block by Hotmail can be challenging, but it's a solvable problem with the right approach. It typically stems from poor sending practices, such as high complaint rates, low engagement, or issues with email authentication. The key to resolving such blocks lies in a combination of immediate action and long-term preventative measures.
By pausing sending, fixing underlying issues like list quality and authentication, and using Microsoft's delisting portal, you can work towards restoring your IP's reputation. However, the ultimate goal is to establish and maintain strong sending hygiene to prevent future blocks. This includes consistent list cleaning, implementing double opt-in, and continuously monitoring your email performance.
Remember, effective email deliverability is an ongoing effort. Proactive monitoring and adherence to best practices will ensure your emails consistently reach the inboxes of your Hotmail (and other) recipients.
