Outlook.com "550 5.7.1 ... part of their network is on our block list (S3150)": how to fix S3150 and S3140

What it means

Microsoft's consumer email service (Outlook.com, Hotmail, Live, MSN) refuses email from your sending IP address because it is on Microsoft's own block list. The bounce usually looks like this:

550 5.7.1 Unfortunately, messages from [203.0.113.10] 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.

The code in brackets (S3150 or S3140) is an internal Microsoft reference for the type of block. Both mean that the IP, or a range of IPs it belongs to, has a poor reputation. "Part of their network" means the block may concern a whole range, not only your IP.

This error is specific to Outlook.com consumer mailboxes. Microsoft 365 business mailboxes use different codes (such as 5.7.606).

Why it happens

• Spam from your IP: complaints from Outlook.com users, or a compromised account or website sending spam.
• Spam from neighbours: on a hosting provider or cloud platform, other customers in the same IP range sent spam.
• Residential or dynamic IPs: IPs from home internet connections are often blocked by default.
• A new IP without any sending history, combined with a sudden volume.
• Missing reverse DNS or authentication, which makes the IP look like an unmanaged server.

How to check

• Send a test email: on the Delivmail home page (/), copy your unique test address and send it a normal email from the same server, tool or mailbox that bounced. Open the report when it is ready.
• Check reverse DNS and blacklists in the report. You can also check the IP on /blacklists.
• Use Microsoft SNDS (Smart Network Data Services, sendersupport.olc.protection.outlook.com/snds). After registering your IP, it shows whether Microsoft sees spam from it and if it is blocked.

How to fix it

1. Stop any unwanted sending first: secure compromised accounts, fix abused forms and empty the mail queue of spam.
2. Make sure the basics are right: a reverse DNS (PTR) record with a real hostname, SPF, DKIM and DMARC passing for your domain.
3. Request delisting from Microsoft. Fill in the Outlook.com sender support form ("Sender Information for Outlook.com Delivery"), linked from Microsoft's troubleshooting pages for senders. Give your IP, the exact bounce, and what you fixed. Microsoft answers by email; if they refuse to fully lift the block, you can reply asking for an escalation with more details.
4. Join SNDS and JMRP. SNDS shows your IP's data. The Junk Mail Reporting Program (JMRP) sends you a copy of the emails Outlook.com users report as spam, so you can remove those people from your lists.
5. If the IP belongs to your host, send them the bounce message: only the owner of the range can clean it up, or they can give you a clean IP.
6. Consider a reputable email service if you send from a cheap VPS range with a poor history.

Check that it is fixed

Send a new Delivmail test and confirm reverse DNS, authentication and blacklists look good. Once Microsoft confirms the mitigation (usually within one or two days after their reply), send a normal email to the Outlook.com or Hotmail address that bounced. Keep volumes low at first: a new wave of complaints brings the block back.