Outlook.com S3150 / S3140: Part of Your Network Is on Our Block List

S3150 (and its sibling S3140) means Microsoft's consumer mail service, the one behind Outlook.com, Hotmail, Live and MSN addresses, has put your sending IP address on its own internal block list and is refusing the connection. It is not a public blocklist you can look up anywhere. You fix it by finding out why the IP looks bad to Microsoft, fixing that, and then asking Outlook.com sender support to review the IP.

The bounce usually looks like this:

550 5.7.1 Unfortunately, messages from [203.0.113.7] 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.

S3140 bounces use the same wording with (S3140) in the brackets.

What it means

Three things are worth reading closely in that bounce.

The IP in square brackets is the one that's blocked. Not your domain, not your content. This is an IP reputation block. Changing the From address or rewriting the email won't get it through.

"Part of their network" is literal. Microsoft sometimes blocks a range rather than a single IP, so a neighbor on the same hosting provider can get you caught. Cheap VPS and cloud ranges are the usual victims.

"Contact your Internet service provider" is boilerplate. If you run your own mail server, you are effectively the provider, and you'll handle the request yourself. If you send through an email service provider (ESP) on a shared IP, the ESP owns the IP and the delisting conversation, so open a ticket with them first.

Microsoft doesn't publicly document how S3140 differs from S3150. In practice the cause and the remedy are the same, so treat them as one problem.

One thing that confuses people: you can see S3150 bounces from hostnames ending in protection.outlook.com, which is also the infrastructure behind Microsoft 365. Don't let that send you to the Microsoft 365 delist portal. The S-codes are the consumer filters talking, and they have their own removal path (more on that below, and in how to get delisted by Microsoft).

Common causes

Most likely first:

1. A new IP with no sending history. Microsoft is cautious about IPs it hasn't seen send before, and more so when they sit in hosting ranges with a poor track record. A freshly provisioned server can hit S3150 on its first send.
2. A compromised account or server. A hacked mailbox, a vulnerable web form or an open relay pumping out spam will burn an IP quickly. If your volume jumped and you don't know why, assume this until you rule it out.
3. Complaints and spam traps. Mail to old, purchased or scraped addresses produces junk reports and trap hits. Microsoft weighs both heavily.
4. A bad neighborhood. Your own sending is fine, but the range around you isn't. This is the "part of their network" case.
5. Missing basics. No reverse DNS (PTR record) on the IP, or no SPF, DKIM or DMARC. These rarely cause S3150 on their own, but they make a weak reputation worse.

How to fix it

1. Collect the evidence. Pull a few full bounces from your logs with timestamps, the sending IP and the recipients. You'll need them for the support request.
2. Find out who controls the IP. If it's a shared ESP IP, stop here and open a ticket with the ESP, including the bounces. They have to make the request, and they may move you to a different pool in the meantime.
3. Stop whatever caused it. Check for compromised accounts, unexpected outbound volume and abused forms. Pause any send to an old or questionable list. Asking for removal while the cause is still running gets you a "no," or a relisting a few days later.
4. Fix the basics. Make sure the IP has a PTR record that resolves to a real hostname, and that hostname resolves back to the IP. Run your domain through the email checker to confirm SPF, DKIM and DMARC are in place.
5. Join SNDS and JMRP. Smart Network Data Services (SNDS) shows Microsoft's view of your IPs: volume, complaint data and filtering results. The Junk Mail Reporting Program (JMRP) sends you a copy of messages Outlook.com users mark as junk. Both are free at the Outlook.com postmaster site. Microsoft recommends both in its own ticket replies.
6. Submit the Outlook.com sender support form. Microsoft's documentation links the Outlook.com delisting form, which needs a Microsoft account to sign in. Include the blocked IP, the full bounce text, your sending domain, roughly how much mail you send to Microsoft addresses, how recipients opted in, and what you changed. Short and factual beats long and indignant.
7. Read the reply carefully. A common response is "Not qualified for mitigation," meaning Microsoft reviewed the IP and won't lift the block yet. That isn't the end. Keep fixing, let clean traffic build history where you can, and reply to the ticket with specifics about what changed.

How to confirm it's fixed

Send a few messages to Outlook.com or Hotmail addresses you control and watch your MTA or ESP logs. A 250 accepted response from Microsoft's servers means the block is lifted. Then check where the messages landed: getting past an S3150 often means landing in Junk for a while, because the IP's reputation is still thin. That's a different problem, covered in why emails go to junk in Outlook and Hotmail.

Keep an eye on SNDS for the next few weeks to see how Microsoft's filters treat your traffic.

How to keep it from coming back

• Warm up new IPs gradually, starting with your most engaged recipients.
• Send only to people who asked for your mail, and remove addresses that bounce or never engage.
• Read your JMRP reports. A spike in junk reports is the earliest warning you'll get.
• Lock down web forms and monitor outbound volume so a compromise is caught in hours, not days.

For the bigger picture of why Microsoft refuses mail, see why Outlook is blocking your emails.

When to get help

If you've fixed the obvious causes and Microsoft keeps answering "not qualified," or the block keeps returning after it's lifted, there's usually a list or infrastructure problem that's hard to see from the inside. That's a good time to bring in a second pair of eyes.

FAQ

Is S3150 a blacklist I can check online?

No. It's Microsoft's own internal list. Public blacklist checkers won't show it, and the only reliable signal is the bounce itself.

How long does it take Microsoft to remove an S3150 block?

Microsoft doesn't publish a timeline for Outlook.com reviews. Plan for some back and forth, especially if the first answer is "not qualified for mitigation."

Will switching to a new IP fix it?

Sometimes briefly, but if the cause is your list or a compromise, the new IP gets blocked too. Fix the cause first.

Does S3150 affect Microsoft 365 business recipients?

S3150 is mostly reported on mail to Outlook.com and Hotmail addresses, though it does sometimes appear on bounces from Microsoft 365 hosts. Microsoft 365 also has its own IP block lists with different codes (5.7.606 to 5.7.649 and 5.7.511) and its own removal process.