Outlook.com S3150: your network is on the block list

Outlook.com has blocklisted the IP address you sent from, not your domain and not this message. The bounce tells you to contact your ISP, which misleads almost everyone who reads it: if you run the sending server, you request delisting yourself through Microsoft's portal. On shared hosting, your host has to ask.

The error

550 5.7.1 Unfortunately, messages from [1.2.3.4] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

Please contact your Internet service provider since part of their network is on our block list (S3140)

What it means

Outlook.com is refusing mail from your IP because it, or a range containing it, is on a Microsoft block list. The S3140 and S3150 suffixes identify which list.

The receiver is refusing your mail because of how your sending IP or domain behaves, not because of this particular message.

Why it happens

The wording tells the recipient to contact their ISP, which misleads almost everyone who reads it — the block is on the sending network, so it is the sender who has to act. It frequently affects a whole hosting provider's range rather than one address.

How to fix it

1. Check whether the blocked IP is shared. On shared hosting the block is often not about you, and your provider has to raise it.
2. Submit a delisting request at sender.office.com with the full bounce text and the IP.
3. If you control the IP, publish valid SPF, DKIM and a PTR record before requesting delisting — a request from an unauthenticated host tends to be refused.
4. Consider whether a dedicated IP or a reputable relay is a better answer than repeatedly delisting a shared one.

Official documentation: Microsoft: use the delist portal.