Urgent Request: Removal from Block List (S3150)

Question

hello. have had serious issues in last 4 weeks sending to outlook.hotmail accounts from my dedicated server. there is absolutely no information from you why this is happening. I get delisted for a couple days and then BOOM, blocked again. mxtoolbox.com shows the IP as very clean. who do I talk to to get information.

 outlook-com.olc.protection.outlook.com[52.101.10.15] said: 550 5.7.1
    Unfortunately, messages from [195.154.178.44] weren't sent. Please contact
    your Internet service provider since part of their network is on our block
    list (S3150). 

Answer 1

Dear Brian Duncan, 

Thank you for your patience and cooperation . The error 550 5.7.1 (S3150) indicates that your IP is on a Microsoft block list due to suspected spam or compromised activity. Even if the delisting portal shows “Nothing was detected,” the block can persist because of historical reputation or network-level issues. Please kindly try other workarounds to see if them might help: 

1. Confirm Technical Configuration 

• Ensure your IP resolves to a valid hostname (Reverse DNS/PTR). 
• Verify SPF, DKIM, and DMARC records are correctly configured. 
• Make sure your HELO/EHLO hostname matches your PTR record. 
• Confirm your mail server supports STARTTLS. 

2. Check for Residual Compromise 

• Review mail logs for unusual sending patterns. 
• Rotate all passwords and enforce strong authentication. 
• Disable any unused accounts. 
• Scan for malware or scripts that could send mail without your knowledge. 

3. Submit a Delisting Request 

• Use the Microsoft Delist Portal. 
• Enter your IP and complete the form. 
• If you receive “Nothing detected,” still submit the request—it refreshes reputation data. 

4. Monitor IP Reputation 

• Check your IP on Microsoft SNDS and JMRP. 
• Avoid sudden spikes in email volume. 
• Ensure all recipients have opted in (no purchased lists). 

5. Contact Microsoft Support 

If all steps fail , please contact Microsoft Support directly at :  

Visit this website:  Contact - Microsoft Support 

1. Type your problems in the search box there. ( For example, Contact support.) 
2. Now click on Get Help and select Contact Support below. 
3. Go to the Products & Services tab and choose the Outlook category 
4. On the Category tab, choose based on your actual problem.
5. Go to Confirm. 
6. Choose to chat with a support representative or request a call back. 

Note: The support contact team only works at certain times, so if the last window shows that it's unavailable, try opening it again the next day. If you prefer phone support, you can leave a phone number. This way, they can call you and help you. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".     

Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.  

Warm regards 

Answer 2

Dear Brian Duncan , 

Thank you for reaching out to the Microsoft Q&A Community. I understand you are experiencing issues with your IP being blocked when sending emails to Outlook/Hotmail accounts. Please follow these steps to resolve the issue: 

1. Verify Mail Server Settings 
   • Ensure valid reverse DNS (PTR) record. 
   • Configure SPF, DKIM, and DMARC records correctly. 
   • Confirm HELO/EHLO hostname matches your PTR. 
2. Check for Compromised Accounts 
   • Review mail logs for unusual activity. 
   • Ensure no accounts are sending spam or bulk mail. 
3. Submit a Delisting Request 
   • Go to Office 365 Anti-Spam IP Delist Portal  
   • Enter your blocked IP address and complete the form. 
4. Monitor IP Reputation 
   • Avoid sudden spikes in email volume. 
   • Ensure all recipients have opted in. 
5. Contact Microsoft Support Directly  
   • Visit this website:  Contact - Microsoft Support  
   • Type your problems in the search box there. ( For example, Contact support.)  
   • Now click on Get Help and select Contact Support below.  
   • Go to the Products & Services tab and choose the Outlook category  
   • On the Category tab, choose based on your actual problem. 
   • Go to Confirm.  
   • Choose to chat with a support representative or request a call back.  

Note: The support contact team only works at certain times, so if the last window shows that it's unavailable, try opening it again the next day. If you prefer phone support, you can leave a phone number. This way, they can call you and help you. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".     

Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread. 

Warm regards

Answer 3

thanks have now resolved. geting mitigated in next 24-48 hours.

Avoid sudden spikes in email volume - I indicated that selling something that is live needs sometimes hundreds of emails within a space of an hour. I can't limit that due to the information needed there and then.

Answer 4

Hi Brian Duncan

It is my pleasure to hear that our response helped resolve your issue! The credit also goes to you.

Is there anything else I can assist you with? If not, please feel free to let me know so I can close this thread. 

Warm Regards