Outlook/Hotmail "550 5.7.1 ... blocked (S3150)" sending IP throttled

What’s actually happening

Mail to outlook.com, hotmail.com, live.com, and msn.com bounces with 550 5.7.1 and an S-code in parentheses. With S3150 some messages may trickle through while others bounce — that's the throttle. With S3140 everything to Microsoft domains is refused outright. Delivery to Gmail and other providers from the same server usually keeps working, which points the finger squarely at your IP's standing with Microsoft.

Common causes

• Your sending IP has poor or no reputation with Microsoft (SNDS / SmartScreen) — common for a brand-new IP or VPS that hasn't been warmed up.
• A spike in volume or a spam/complaint signal tripped Microsoft's filters. Recipients hitting "junk" feeds straight back into your reputation.
• You're on a shared host or cloud range (AWS, DigitalOcean, OVH) where a neighbor's bad behavior got the whole subnet flagged — note the message says "part of their network."
• Missing or misaligned SPF/DKIM/DMARC and no reverse DNS (PTR) on the sending IP, which Microsoft weighs heavily.
• A compromised account or open relay on your server quietly sending spam, which Microsoft detected before you did.

How to fix it

1. Tell S3140 from S3150 and act accordinglyS3150 means throttled — you're not fully blocked, so slowing your send rate and fixing reputation may clear it on its own. S3140 means a full block; that one needs a delisting request. Check which code is in your bounce before deciding how hard to push.
2. Submit a delisting request to MicrosoftGo to https://sender.office.com (the Sender Support / delisting form), enter the exact blocked IP and an admin email on the sending domain, and submit. Microsoft typically responds within 24 hours. This is the primary remedy for S3140 and a hard-stuck S3150.
3. Fix authentication and reverse DNS firstBefore or alongside delisting, make sure the IP has a valid PTR record matching your sending hostname, and that SPF, DKIM, and DMARC all pass. Microsoft often won't keep an IP delisted if these are missing — ask your host to set the PTR if you can't.
4. Enroll in SNDS and JMRPSign up for Microsoft's Smart Network Data Services (sendersupport.olc.protection.outlook.com/snds) to see complaint and spam-trap data for your IP, and join the Junk Mail Reporting Program (JMRP) to get notified when Outlook users mark you as spam. You can't fix reputation you can't see.
5. Slow down and warm the IPCut your send rate to Microsoft domains, prune dead and complaining addresses from your list, and ramp volume gradually over days. If you're on a noisy shared cloud IP, the durable fix is moving to a dedicated IP with clean reputation.