Could not be delivered because of

Question

Hi,

Can you check if our IP is blacklisted ?

This is the message we get:

Could not be delivered because of

 

Server refused mail at MAIL FROM - 550 5.7.1 Unfortunately, messages from [[Moderator note: personal info removed]] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to [http://mail.live.com/mail/troubleshooting.aspx#errors]. [Name=Protocol Filter Agent][AGT=PFA][MxId=[Moderator note: personal info removed]] [[Moderator note: personal info removed].prod.outlook.com] 2026-02-17T09:33:44.150Z [Moderator note: personal info removed]]

Kind regards, [Moderator note: personal info removed]

Answer 1

Please note that Q&A forum is a public platform, and moderators will modify the question to hide personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data.

Dear Danny Buekers

Thank you for reaching out to the Microsoft Q&A forum. I understand how confusing and frustrating this situation can be.

Please note that as a forum moderator, I don’t have the ability to remove blocks or make changes on your behalf. My role is limited to providing guidance and directing you to the appropriate escalation channels.

The recommended next step is to escalate the issue for a manual review by the Outlook Deliverability Support Team. You can do this by submitting a request through the following link: https://olcsupport.office.com/ When submitting the request, please ensure you bypass the automated portal and file a support ticket directly with the deliverability engineers.

• Issue Title: "IP blocked with error S3150 - Mitigation Request"
• IP Address: Your IP address
• Error Message: Paste the full 550 5.7.1 ... (S3150) error log.

Alternatively, you may open a support ticket directly from your Microsoft 365 tenant via the Admin Center or contact Microsoft Support through phone call. If you’re unable to sign in, please refer to this Microsoft Q&A article for the correct approach: https://learn.microsoft.com/en-us/answers/questions/5769495/signing-in-to-our-microsoft-365-admin-portal

I truly hope your issue is resolved as quickly as possible. While I wish I could take direct action for you, moderators do not have the ability to remove or override deliverability blocks.

References:

• https://learn.microsoft.com/en-us/answers/questions/5735325/spam-ip-not-blocked-but-mails-refused-due-to-block
• https://learn.microsoft.com/en-us/answers/questions/5531091/urgent-request-removal-from-block-list-(s3150)

Thank you for your understanding. If you receive any updates, please feel free to reply here.

---

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

Answer 2

The error message you received indicates that the IP address [Moderator note: personal info removed] is likely on a block list, which is preventing your emails from being delivered. This is a common issue when the sending IP address is associated with spam or other malicious activities. To resolve this, you can take the following steps:

1. Check IP Blacklist Status: Use online tools to check if your IP address is listed on common blacklists. There are several services available that can help you verify this.
2. Contact Your ISP: Since the error message suggests contacting your Internet Service Provider (ISP), it’s advisable to reach out to them. They may be able to assist you in resolving the issue or provide more information about why your IP is blocked.
3. Delist Request: If your IP is indeed blacklisted, you may need to follow the procedures provided by the blacklist to request removal. This often involves ensuring that your email practices comply with best practices to avoid future listings.
4. Monitor Email Practices: Ensure that your email sending practices are compliant with anti-spam regulations and best practices to prevent being blacklisted again in the future.

For more detailed guidance, you can refer to the Microsoft documentation regarding blocked senders and delisting procedures.

---

References:

• External senders - Use the delist portal to remove yourself from the blocked senders list and address 5.7.511 Access denied errors
• External senders - Troubleshoot email sent to Microsoft 365