IP blocked with error S3150 – Mitigation Request (Ref: [Moderator Note: Personal Info Removed])

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IP blocked with error S3150 – Mitigation Request (Ref: [Moderator Note: Personal Info Removed])

Unigas Alicante 0

Hello, Our mail server's IP address (xx, hostname box.unigasalicante.com, sending domain unigasalicante.com) is being blocked by Outlook.com/Hotmail with the following error: 550 5.7.1 Unfortunately, messages from [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] We submitted a delisting request via the "Sender Information for Outlook.com Delivery" form 48 hours ago. Support request number: [Moderator Note: Personal Info Removed] We have not received any update yet. SPF, DKIM and DMARC are all correctly configured and passing (confirmed via Google's DMARC aggregate reports), and the IP is not listed on Spamhaus, SpamCop, Barracuda, SORBS or other major DNSBLs. This is a small business mail server sending legitimate transactional emails only. Could someone please help check or escalate the status of this request? Thank you.

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Answer 1

Hi @Unigas Alicante

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.

I’m sorry to hear that your legitimate transactional emails are still being blocked with the S3150 message.

Since you have already submitted the Outlook.com Sender Support form, please check the mailbox used for the request, including the Junk/Spam folder, for an automated response. If the message asks you to reply without changing the subject or body, or to follow a verification link, please complete that step so the request can proceed.

If no response can be found, please submit a new request through the Microsoft Support form and include the affected IP address and sending domain, the complete rejection message/NDR, recent sample timestamps, approximate sending volume, and the corrective actions already taken.

Please note that this is a user-to-user support forum. Moderators, contributors including external Microsoft employees cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas only.

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