Delivery failure to Outlook.com - IP XXX - Error S3150

German Palomares 0

I am writing to request a mitigation for my dedicated IP [XXX], which is currently being blocked with the error code S3150 (550 5.7.1).

I recently acquired this dedicated IP through Brevo to send legitimate transactional and marketing emails to our subscribers. We follow all best practices, including:

Correct SPF, DKIM, and DMARC records.

Regular list cleaning and double opt-in.

Low complaint rates.

The specific error received is: 550 5.7.1 Unfortunately, messages from [XXX] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

How can I reach out Azure support to review this IP and delist it? If there are specific issues, please let me know so I can address them immediately.

Thank you for your assistance.

Shubham Sharma 17,930 Reputation points Microsoft External Staff Moderator

2025-12-19T09:30:08.9566667+00:00
Hey German! It looks like you’re facing some trouble with your dedicated IP getting blocked with error S3150 when trying to send emails to Outlook.com. Here are a few steps you might want to consider:

Check Your Setup: It's great that you're following best practices like setting up SPF, DKIM, and DMARC. Make sure these records are correctly configured and accessible through tools like MXToolbox.

Gather Error Details: Error code S3150 indicates your IP might be on a block list due to having a poor reputation. Keep a note of the full error message as it will be helpful when seeking support.

Contact Azure Support: You can reach out to Azure support for assistance with delisting your IP. The common route is to open a support ticket through the Azure portal. Provide them with the full details of your issue, including the error code and any relevant email headers if possible.

Delist Request: In case the IP is indeed blocked on Microsoft's end, you can also submit a request through the delist portal (you might find prompts to address certain issues directly there). Visit the link provided in the documentation to get more details on the process.

Use Authenticated SMTP Relay: Consider using an authenticated SMTP relay service for better deliverability. Azure recommends using services on port 587, which aren’t blocked.

Test Your Connectivity: Make sure there aren’t any other connectivity issues on port 25. You can run diagnostic tests in Azure to ensure your subscription type allows outbound SMTP connectivity based on Microsoft’s policies.

Hope this helps! If you have more specific details or further questions, feel free to ask.

Follow-Up Questions:

Could you provide more details about your current email sending configuration?

What subscription type are you using for your Azure services?

Have you tested your IP against any blocklist lookup services to see its reputation?

Are there any other error codes showing besides S3150?

References:

Fix Email Delivery issues in Microsoft 365

Limitations and recommendations for sending outbound email over SMTP port 25

Use the delist portal to remove yourself from the blocked senders list

Note: This content was drafted with the help of an AI system. Please verify the information before relying on it for decision-making.

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