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550 5.7.1 S3150: Outlook.com blocked your IP

550 5.7.1 (S3150)Outlook.com network block listPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com

Example
550 5.7.1 Unfortunately, messages from [203.0.113.90] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

What it means

S3150 comes from Microsoft’s consumer mail service (outlook.com, hotmail.com, live.com, msn.com), not from Exchange Online tenants. It says your IP, or a range around it, is on the Outlook.com block list, and suggests you contact your Internet service provider.

Because it is consumer side, the sender.office.com portal for Microsoft 365 is not the main route. Outlook.com has its own sender support form, plus SNDS and JMRP for monitoring. S3140 is a close sibling with the same wording and a different block reason code.

The sender, or whoever owns the IP, must act. If the IP is on a residential or dynamic range, the honest fix is to stop sending direct from it.

Likely causes

  • Spam came from your IP or from neighbours in the same range.
  • The IP is in a dynamic or residential block that should not send mail directly.
  • Complaints from Outlook.com users about your mail rose sharply.
  • You started sending from a new IP without warm up.

How to fix it

  1. Make sure no spam is going out from your server, and fix reverse DNS, SPF, and DKIM.
  2. Submit a request through the Outlook.com sender support form, giving the IP and the full NDR text.
  3. Register the IP with SNDS to see its status and join JMRP for complaint feedback.
  4. If you are on a home or dynamic connection, relay through your provider’s smarthost or a mail service.
  5. If Microsoft declines mitigation, keep the IP clean and ask again later, or move to a better IP.

Checks that apply

  • Blacklist Check Confirms whether your IP is also listed on public DNSBLs, which often goes with an Outlook.com block.
  • Reverse DNS (PTR) A generic or missing PTR record makes the IP look residential.
  • Email Deliverability Summarizes the authentication and reputation signals Outlook.com weighs.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

How long does Outlook.com mitigation take?
Support usually answers within a day or two. Approval is not guaranteed, and some IPs only qualify after a period of clean sending.
Does an S3150 block affect Microsoft 365 business mailboxes?
Not directly. It applies to Outlook.com consumer domains, although poor IP reputation often shows up in both.

Microsoft 365 delist portal Outlook.com SNDS

All 191 bounce codes