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How to Resolve Microsoft Email Server IP Blocking Error S3150
Question details
The user needs to unblock an SMTP server IP address that has been blocked by Microsoft, which results in email delivery failure with error S3150.
- Product
- Microsoft Exchange Server
- Device & OS
- not provided
- Scenario
- Attempting to send emails using a new or failover IP address that Microsoft flags as low reputation, despite having correct DNS configurations.
- Observed behavior
- Microsoft blocks the SMTP server IP address and returns an S3150 error, preventing emails from being delivered.
Verify that your server's reverse DNS (rDNS), SPF, DKIM, and DMARC records are correctly configured and propagated before submitting a delisting request to Microsoft.
Open a Support Ticket in Microsoft 365 Admin Center
The recommended method for Microsoft 365 subscribers to request backend investigation and IP delisting.
Because this issue concerns a blocked IP address and requires backend investigation, it cannot be resolved through the public Microsoft Community. Administrators must contact technical support directly through the Admin Center.
Navigate to the Microsoft 365 Admin Center using your global administrator credentials.
Click on the 'Support' tab in the left-hand navigation menu and select 'Help & support'.
Open a new service request. Provide the specific blocked IP address, server details, and mention the S3150 error code to initiate the investigation.
Submit a Microsoft IP Delisting Request (For Non-Subscribers)
For organizations without a Microsoft 365 subscription, use the public Anti-Spam IP Delist Portal.
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Frequently Asked Questions
Why is my IP blocked even with correct SPF, DKIM, and DMARC records?
Microsoft may preemptively block new or low-reputation failover IP addresses as a precaution against spam. Correct authentication records alone do not bypass these reputation-based filters.
How long does the Microsoft delisting process take?
Once your request is submitted and approved, it can take anywhere from a few hours to 48 hours for the block to be fully lifted across Microsoft's global network.
Can I resolve error S3150 in the Microsoft Community forums?
No. Because this issue involves backend investigation of a specific blocked IP address, it cannot be resolved publicly. You must use the official delist portal or contact Microsoft technical support directly.
