550 5.7.1 S3140: Outlook.com block list rejection
550 5.7.1 (S3140)Outlook.com block list (S3140)Permanent: retrying will not helpFrom Microsoft 365 and Outlook.com
550 5.7.1 Unfortunately, messages from [198.51.100.14] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140).
What it means
An S3140 NDR uses almost the same sentence as S3150, so people often mix them up. The bracket code tells Outlook.com support which rule blocked the IP, and you should quote it exactly. Like S3150, it only appears for consumer recipients on outlook.com, hotmail.com, and live.com.
In practice S3140 is commonly reported by senders on cloud or hosting IPs that Outlook.com treats as risky, and by servers with sudden jumps in volume. The remedy path is the same: clean up, then ask for mitigation through the Outlook.com sender support form.
The person who controls the IP has to act; your ISP or host may need to file the request if the block covers their range.
Likely causes
- Your IP sits in a cloud or VPS range that saw abuse from other customers.
- Your server sent a burst of mail with weak engagement or many unknown recipients.
- Reverse DNS or authentication was missing, making the IP look untrusted.
How to fix it
- Verify the server is not compromised and that only your own systems can send through it.
- Set a proper PTR record and publish SPF and DKIM for every domain you send as.
- File a request with the Outlook.com sender support form quoting S3140, the IP, and the full bounce.
- Enroll the IP in SNDS and JMRP, then watch the data for a few weeks.
- If you are on a cloud provider that discourages direct mail, relay through a reputable sending service.
Checks that apply
- Reverse DNS (PTR) Shows whether the IP still has a generic cloud provider PTR name that hurts trust.
- ASN Lookup Shows which provider network the IP belongs to, explaining a range wide block.
- Blacklist Check Finds other listings you should clear before asking Outlook.com for mitigation.
FAQ
- Is S3140 handled differently from S3150?
- The removal route is the same sender support form; quote the exact code so support applies the right rule.
- Can the recipient whitelist me?
- Outlook.com users can mark mail as not junk, but they cannot override an IP block at connection time.