s3150 block list removal

Hi @Mike Wilding

Thank you for bringing the S3150 blocklist issue to our attention. We understand how important reliable email delivery is for your business, and we’re here to help you resolve this as efficiently as possible. 

This error typically indicates that the IP address used to send your emails has been temporarily blocked by Microsoft due to suspected spam-like activity or poor sender reputation.  

Below are the steps we recommend addressing the issue: 

Submit a Delisting Request via Microsoft Sender Support 

This is the most direct way to have your IP removed from the blocklist. Please submit a request through Microsoft's official Sender Support portal: https://sender.office.com 

When submitting your request, include the following: 

• The blocked IP address. 
• The full Non-Delivery Report (NDR) or message headers showing the rejection. 
• Confirmation that the IP is not listed on any public blocklists. 
• A brief explanation of the corrective actions you’ve taken to improve your sender reputation. 

Important: Be sure to complete the email verification step after submitting the form to finalize the process. 

Check Your SPF, DKIM, and DMARC Configuration 

Properly configured authentication records are essential for building and maintaining a strong sender reputation. Ensure your domain has valid SPF, DKIM, and DMARC records aligned with your sending setup. 

• SPF: For Microsoft 365, the recommended SPF record is 

v=spf1 include:spf.protection.outlook.com -all. If you use other services, ensure their IP addresses are also included. 

• DKIM: Confirm that your outgoing messages are signed with a valid DKIM. 
• DMARC: Check that your DMARC policy is configured correctly. At a minimum, setting it to p=none allows you to monitor authentication without affecting delivery. 

You can check your current domain setup using tools such as: https://mxtoolbox.com 

Please note: This link leads to a third-party website not managed by Microsoft. Please review their terms and privacy policy before use. 

Review Sending Infrastructure  

Check whether your email traffic is being routed through on-premises servers or third-party relays. In these cases, the IP address evaluated by Microsoft may differ from your primary one, it’s important to identify the exact IP that was blocked. 

Analyze Message Headers 

Providing full message headers from a bounced email helps Microsoft pinpoint the exact cause of the block. 

If you're unsure how to access message headers, follow this Microsoft guide: View internet message headers in Outlook - Microsoft Support 

Then, paste the headers into the Message Header Analyzer tool. 

Microsoft typically responds to delisting requests within 24 to 48 hours. If the issue persists beyond that timeframe or if it requires urgent resolution, we recommend reaching out to Microsoft Support directly via the Microsoft 365 Admin Center (for tenant administrators). 

Alternatively, if you're using a third-party email service provider, you may also want to reach out to their support team, as they often have direct channels to help resolve sender reputation or IP blocking issues. 

If you'd like me to help review your setup or go through any part of the process together, just let me know. I'm more than happy to assist. 

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