550 5.7.1 "Messages Weren't Sent ... Block List (S3150)": Outlook.com Fix
What the Outlook.com S3150 and S3140 bounce means, why it names your provider's network, who can request delisting, and what to check on your domain.
This bounce comes from Microsoft's consumer mail service (Outlook.com, Hotmail, Live and MSN addresses): "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)." It is an IP block, not a judgement on your domain or your message. The IP in the bounce is the server that tried to deliver, and the code at the end says whether Microsoft blocked that one IP or the range around it.
Quick answer
If you send through a hosted mail provider, forward the full bounce to their support; the IP belongs to them and only they can request removal. If the IP is your own server or VPS, submit Microsoft's sender support form for Outlook.com, enrol the IP in SNDS and JMRP, and fix whatever caused the listing before you ask. While the block lasts, mail to Outlook.com addresses from that IP will bounce; mail to every other provider is unaffected.
What the codes mean
- S3150: part of the network the IP sits in is on the block list. Often a neighbouring IP in the same range caused it; shared hosting and small VPS providers see this most.
- S3140: the specific IP is on the block list, usually after spam or a high complaint rate from it.
- Both are Microsoft's own lists, not Spamhaus or another public list, so a clean result on the public blocklists does not mean Outlook.com is clear.
- The related "550 5.7.1 Service unavailable, Client host [IP] blocked using Spamhaus" comes from Microsoft 365 business tenants and points at a public list instead.
What to do, in order
- Copy the IP from the bounce and check whether it is yours. A hosted mailbox provider or an email API sends from its own IPs; a VPS, a web host or an office mail server sends from yours.
- If it is a provider's IP, send them the bounce. Reputable providers watch for S3150 and already have a delisting contact. There is nothing to change on your domain.
- If it is yours, look for the cause first: a compromised account sending spam, a web form being abused, a script retrying bounced addresses, or a bulk send to an old list. Microsoft rejects delisting requests that do not fix the cause.
- Enrol the IP in Smart Network Data Services (SNDS) to see Microsoft's view of its traffic and complaint rate, and in the Junk Mail Reporting Program (JMRP) to receive copies of complaints.
- Submit the Outlook.com sender support form with the IP and the bounce text. A reply usually arrives within a few days; some requests are refused the first time and need a second with more detail.
- Meanwhile, send mail to Outlook.com recipients from a different IP if you have one, or through a hosted provider.
If you are on OquMail
Your mail leaves from OquMail's servers, so the IP in the bounce is not something you control. Paste the bounce into a support message and the team handles the request with Microsoft. Check your own side as well: the delivery log shows whether only Outlook.com recipients are failing (an IP block) or everyone is (a domain or content problem), and the two are fixed differently.
Common questions
My domain reputation is fine. Why am I blocked?
Because the block is on the IP range, not the domain. S3150 in particular lands on senders who did nothing wrong but share a range with someone who did.
How long does delisting take?
Typically 24 to 72 hours after Microsoft accepts the request. Blocks are also lifted automatically over time when the range stops sending spam, but that can take weeks.
Mail reaches Outlook.com but goes to Junk. Is that S3150?
No. A block produces a bounce. Junk placement is reputation and content, covered in why mail lands in Junk at Outlook.com.