550 5.7.1 (S3140) — Messages from your IP weren't sent (Outlook.com)
Retrying will not help. The message will not be delivered.
The reply
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 (S3140).Your sending IP is on Outlook.com's block list. The advice in the reply — contact your ISP — is wrong for anyone running their own mail server: the request has to come from whoever controls the IP, and there is a form for exactly that.
What it means
Outlook.com maintains a block list of its own that has nothing to do with Spamhaus or any public list, so an IP can be clean everywhere else and still be refused here. New IPs land on it routinely, because Microsoft treats an address with no history as untrusted by default rather than neutral. Delisting is a form submission, and it is usually granted — but it will not hold unless the underlying reputation improves, and a second listing is harder to clear than the first.
Why it happens
- A new or recently reassigned IP with no sending history at Microsoft.
- The IP sits in a range Microsoft distrusts — many cloud and VPS ranges are treated this way by default.
- Spam complaints from Outlook.com recipients about earlier mail from this IP.
- A previous tenant of the same IP burned it before you were assigned it.
- A volume spike from an IP Microsoft had seen sending little.
- Missing or mismatched reverse DNS on the sending IP.
What to do
- Submit the delisting request at sendersupport.olc.protection.outlook.com/pm/ — from the party that controls the IP, not the ISP. It is free and usually answered within a day.
- Before requesting, fix what would get you re-listed: reverse DNS that resolves and matches your HELO, SPF, DKIM and a DMARC policy.
- Enrol the IP in SNDS (Smart Network Data Services). It shows the complaint rate and trap hits Microsoft sees from your IP — the only visibility you get here.
- Join JMRP, Microsoft's feedback loop, so junk reports reach you and you can suppress those recipients instead of learning about them as a block.
- Ramp volume gradually after delisting. A fresh delisting plus a full-rate send is how you get listed again.
- Check the IP is not in a range known for abuse; if it is, a dedicated address from a provider with a better neighbourhood is the real fix.
Questions
The message says to contact my ISP.
That text is aimed at home users on a residential connection. If you run the mail server, you are the party who must file the delisting request — use the sender support form directly.
I was delisted and it came back.
Delisting clears the symptom; it does not change the reputation that caused it. Set up SNDS and JMRP so you can see what Microsoft sees, and fix the complaint source before requesting again.
My IP is clean on every blocklist.
Microsoft's list is entirely its own. Being clean on Spamhaus, Barracuda and SpamCop says nothing about whether Outlook.com will accept you.