Request to unblock our email server IP

Question

Request to unblock our email server IP

Darko Kraus 0

Hello,

It was brought to our attention that our email server IP 50.77.xx.xx was being blocked by filtering service for domain outlook.com. Our email server is not blocked or marked for spam by any of the providers from mxtoolbox.com as it can be verified here:

"https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a50.77.xxx.xx&run=toolpage"

The following error is received when we attempt to send message to ******@outlook.com:

<<< 550 5.7.1 Unfortunately, messages from [50.77.xxx.xx] 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=11Bxxxx98] [DUxxxx01.eurprd03.prod.outlook.com 2026-02-13T00:41:49.983Z 08xxxxxF53] 554 5.0.0 Service unavailable

Please investigate and unblock our IP.

Thank you.

Andrian 0 Reputation points

2026-02-13T11:39:56.35+00:00

Dealing with the S3150 error is a massive headache because Outlook's internal reputation filters are way more aggressive than the standard blacklists you'll find on MXToolbox. Even if you get delisted, it's worth checking if something in your actual email headers or content is triggering their sensors. I usually run a test through https://unspam.email/ to see if there's a hidden configuration issue or a "spammy" signature detail that Microsoft's bots are hating on...

Anyway, definitely keep hounding that delist portal until a human actually looks at it.
Darko Kraus 0 Reputation points

2026-02-14T05:05:06.47+00:00

Hello,

Thank you for the update. I sumitted a ticket with Microsoft and they removed the block for our SMTP IP address. Even though we never get blocked, I will check email headers to see if there is something unusual in it.

Thank you.

2 answers

Your answer

Andrian 0 Reputation points

2026-02-13T11:39:56.35+00:00

Dealing with the S3150 error is a massive headache because Outlook's internal reputation filters are way more aggressive than the standard blacklists you'll find on MXToolbox. Even if you get delisted, it's worth checking if something in your actual email headers or content is triggering their sensors. I usually run a test through https://unspam.email/ to see if there's a hidden configuration issue or a "spammy" signature detail that Microsoft's bots are hating on...

Anyway, definitely keep hounding that delist portal until a human actually looks at it.
Darko Kraus 0 Reputation points

2026-02-14T05:05:06.47+00:00

Hello,

Thank you for the update. I sumitted a ticket with Microsoft and they removed the block for our SMTP IP address. Even though we never get blocked, I will check email headers to see if there is something unusual in it.

Thank you.

Answer 1

Please note that our forum is a public platform, and we will modify your question to hide your 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.

Hi @Darko Kraus

Thanks for reaching out to the Microsoft Q&A forum.

To resolve the error code S3150 when sending messages to Outlook.com, please submit a support request through the following link: https://olcsupport.office.com/

The system will review your case and assist with the unblock process, as outlined in the official troubleshooting documentation: Troubleshooting

User's image After submitting the ticket, if you receive an automated email stating that your IP is not blocked even though the issue is still occurring, please reply directly to that email. This will ensure your case is forwarded to a human agent who can look into the situation more carefully.

For a similar situation reported by another user, you may also find this discussion helpful: S3150 filtering. - Microsoft Q&A

Please note that the unblock process can only be handled through a support request submitted via the link above. It cannot be completed through general forum platforms.

I hope this helps you move forward. If you have any updates, please feel free to reach out.

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

To request the unblocking of your email server IP (50.77.224.12) from Microsoft's filtering service, you can use the Anti-Spam IP Delist Portal. This self-service portal allows you to request removal from the Microsoft 365 blocked senders list. If you receive a non-delivery report (NDR) indicating that your IP is blocked, you should follow the instructions provided in the NDR message.

You can also send an email to delist@microsoft.com, including the full NDR code and your IP address. Microsoft typically responds within 48 hours with the next steps regarding your request.

Make sure to verify that your server is not advertising itself as a non-routable IP and that it passes reverse-DNS lookups, as these can also lead to blocking issues.

References:

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Request to unblock our email server IP

2 answers

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