550 5.7.1 Outlook/Hotmail Sending IP Blocked (Reputation Blocklist, OU-002)

What’s actually happening

Mail to outlook.com, hotmail.com, or live.com bounces with 550 5.7.1 and an OU-prefixed code such as OU-002. Microsoft has blocklisted your sending IP or domain on reputation grounds and is refusing the connection outright. This is different from the S3150 / temporary throttle, where mail is merely deferred — OU-002 is a hard block until you get delisted. The bounce points at mail.live.com troubleshooting and effectively says your IP sits on their block list.

Common causes

• The sending IP accumulated spam complaints or hit Microsoft spam traps and dropped in SmartScreen/SNDS reputation
• A sudden volume spike from a new or cold IP that Microsoft hadn't seen sending before
• A shared or neighboring IP on the same range got listed, dragging your reputation with it
• Missing or failing SPF/DKIM/DMARC on outbound mail, which Microsoft weighs heavily
• A compromised account or open relay sending spam through your infrastructure

How to fix it

1. Submit the IP to the delist portalGo to sender.office.com (the Office 365 anti-spam / delist portal) and file a delisting request for the blocked IP. For OU-002 you usually must explain the root cause; a request with no fix behind it gets denied or re-listed.
2. Fix the trigger before requesting reviewStop the bad sending first — purge dead addresses, kill any spam source, lock down a compromised account. Microsoft re-lists IPs that get delisted and immediately reoffend.
3. Authenticate all outbound mailPublish and pass SPF, DKIM, and DMARC for the sending domain. Microsoft leans on authentication for reputation; unauthenticated mail from a flagged IP won't recover.
4. Enroll in SNDS and JMRPRegister the IP in Smart Network Data Services (SNDS) for spam-trap and complaint data, and join the Junk Mail Reporting Program (JMRP) feedback loop to catch complaints before they tank reputation again.
5. Warm the IP and throttle volumeAfter delisting, ramp send volume gradually rather than resuming full blast. A cold IP that immediately sends high volume looks exactly like the pattern that got it blocked.