Outlook.com S3150 'Part of Their Network Is on Our Block List': What It Means and Who Can Fix It

550 5.7.1 Unfortunately, messages from [203.0.113.25] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). This bounce comes from Outlook.com, Microsoft's consumer mail service, and means it has blocked the IP or IP range your mail left from. It hits every message to Outlook.com, Hotmail, Live and MSN addresses from that IP, whatever the content. The bounce tells you to contact your "Internet service provider" because the fix belongs to whoever owns the sending IP. If you send through an email service provider, a cold email platform or a hosted server, that is them. They submit a request through Microsoft's Outlook.com sender support form, after the cause is fixed.

Reading the bounce

The text is the same in every report we found, including a Microsoft Q&A thread from February 2024 and another from February 2026 (both read 2 November 2026):

550 5.7.1 Unfortunately, messages from [IP] 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.

• 550 5.7.1 is a permanent policy rejection. Your server will not retry.
• The IP in brackets is the server that handed over the message. That is the IP Microsoft blocked, or part of the range it blocked.
• "Part of their network" is the key phrase. The block can cover a range, so you can be listed because of neighbours on the same network.
• S3150 is Microsoft's internal reference. Microsoft does not publish a definition for it, and posters on its own forum note that the troubleshooting link in the bounce does not mention it. S3140 appears in the same message and is handled the same way.

Who owns the IP?

This decides who acts. Run the IP through the reverse DNS lookup and the which ESP sent this email tool if you have a copy of the message.

You send through	Who owns the IP	Who submits the request
An ESP or marketing platform	The provider	The provider. Send them the bounce
A cold email platform's shared sending	The platform	The platform
Your own server on a VPS or cloud host	You, on the host's network	You, and the host if the whole range is listed
Google Workspace inboxes	Google	Unusual. Check the bounce again
Microsoft 365 inboxes	Microsoft	Unusual. Check the bounce again
An office or ISP connection	The ISP	The ISP

A question on Microsoft's forum in February 2026 came from a marketer getting conflicting answers about whether the "provider" was the company's IT team or the marketing automation platform sending on a dedicated IP. The answer is whoever controls that IP. For a dedicated IP at a platform, that is normally the platform's deliverability team.

Which Microsoft form to use

There are two separate Microsoft processes, and S3150 threads often mix them up.

Process	For	Code you will see
Office 365 Anti-Spam IP Delist Portal, sender.office.com	IPs blocked from sending to Microsoft 365 business tenants	550 5.7.606 to 5.7.649
Outlook.com sender support request form	IPs blocked by Outlook.com, the consumer service	S3150, S3140 and other Outlook.com codes

Microsoft's delist guide (read 2 November 2026) says: "The delisting form for Outlook.com, the consumer service can be found here. Be sure to read the FAQ first for submission direction." Use that form for S3150. If your block is on Microsoft 365 instead, our 550 5.7.606 guide covers the portal step by step.

Before you submit: fix the cause

Some senders on Microsoft's forum report requests answered with "not qualified for mitigation". That usually means the IP's recent traffic still looks bad to Microsoft. Fix the cause first.

Likely cause	What to check
Mail to old, bought or unverified lists	Recent bounce rates to Outlook.com and Hotmail
Spam complaints	Consumer recipients marking mail they did not expect as junk
Sudden volume from a new IP	A new server or dedicated IP sent at full speed
A compromised account or form	Outbound logs for mail you did not write
A neighbour on the same range	Other customers of a cheap host or shared pool
Missing reverse DNS or authentication	PTR record, SPF, DKIM and DMARC

The IP blacklist check shows whether the same IP is on Spamhaus, Barracuda or others, which often happens at the same time. The Microsoft sender requirements checker confirms your domain meets Outlook.com's authentication rules, which Microsoft has enforced for high-volume senders since May 2025.

What to put in the request

• The full bounce text, with the S3150 code and the IP.
• Timestamps and the server reference from the end of the bounce.
• What the IP sends (transactional, newsletter, one-to-one business mail) and roughly how much a day.
• What you changed: list cleaned, compromised account closed, volume reduced, authentication fixed.

Keep it short and factual. A request that admits a cause and shows the fix is easier to approve than one that says nothing changed.

While you wait

• Pause sends to consumer Microsoft domains from the blocked IP: outlook.com, hotmail.com, live.com, msn.com and their country versions. Our Hotmail, Outlook, Live and MSN domains list has them all. The remove emails by domain tool splits them out of a CSV.
• Keep sending to everyone else at normal pace if their bounces are clean. S3150 is an Outlook.com block, not a general one.
• Do not switch IPs to dodge it. If the cause is your list or your volume, the new IP will follow.
• Clean the list before you resume, so the first sends after delisting do not undo it.

Cold email and consumer Microsoft addresses

Most cold email goes to business addresses, and those on Microsoft 365 are not affected by S3150. If your list has many outlook.com and hotmail.com addresses, ask whether they belong in a cold campaign at all. Personal inboxes are the least likely to expect a business pitch, and a junk click there counts against the sending IP. Our why Outlook leads come back risky guide covers how to route them, and emails going to junk in Outlook covers placement once you are delivering again.

Where SimpleVerifier fits

Verification cannot delist an IP. Outlook.com, Hotmail, Live and MSN addresses come back risky with "Provider hides mailbox status" in SimpleVerifier, as our how it works page explains, so you can see them as their own segment and decide how to handle them. For everything else, SimpleVerifier runs a live mailbox check and returns safe to send, send with care, check again later or do not send, with the reason, and catch-alls stay risky, never guessed valid.

The first 100 checks are free with a Google sign-in on the verify page. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.