How does Microsoft’s SNDS show throttling vs blocking?

You log into Outlook's Smart Network Data Services (SNDS) dashboard, pull up your sending IPs, and see a mix of green, yellow, and red. Now what? The color alone doesn't tell you what's actually happening to your mail. Here's how to read what Microsoft is really saying.

Green means you're in good standing. Mail is being accepted at normal speed. No red flags from Microsoft's filters. Keep doing what you're doing.

Yellow means you're being throttled. Microsoft is accepting your mail, but it's slowing down the connection. Your sending server gets deferred responses (think 421 or 450 codes) in the SMTP log, and delivery takes longer than normal. The mail isn't rejected. It's queued and eventually delivered, just at a crawl. Throttling usually means Microsoft is watching your IP and isn't ready to trust it fully yet. It's a warning, not a sentence.

Red means you're being blocked. Mail to Microsoft is rejected outright, not deferred. Your bounce messages will reference a policy violation and often include a link to Microsoft's sender support portal. Nothing is getting through until the issue is resolved. That's the key practical difference from throttling: deferrals retry automatically, rejections don't deliver at all.

The color status comes from Microsoft's filters reading three signals about your IP over a rolling data window.

• Spam trap hits. SNDS shows how many messages from your IP landed on Microsoft-operated trap addresses. Even a handful of trap hits can push you from green to yellow fast. High trap counts usually point to a stale or unvalidated list.
• Complaint rate. This is the percentage of your mail that recipients flagged as spam inside Outlook, Hotmail, or Microsoft 365 inboxes. Microsoft's threshold is famously tight. If your complaint rate climbs above roughly 0.3% on Microsoft traffic, expect yellow or red quickly.
• Filter result. SNDS shows the percentage of your mail that Microsoft's filters marked as spam, regardless of what the recipient did. A high filter result is a strong signal that your content or reputation is triggering Microsoft's automated systems.

One thing worth knowing: SNDS is per-IP, not per-domain. If you're sending from multiple IPs (shared or dedicated), each one has its own status. An IP that looks fine in your dashboard might be clean while another is sitting at red. Always check every IP you're actually sending from.

But if you're throttled (yellow), the practical fix is to slow your sending volume on that IP, clean up whatever list problem drove the trap hits or complaints, and give Microsoft time to re-evaluate. If you're blocked (red), you'll likely need to go through Microsoft's Junk Mail Reporting Program (JMRP) and their sender support process to start the delisting path. Neither is fun, but throttling is the far easier of the two to recover from.

If your list feels like the root cause, that's worth fixing before you do anything else. A list clean can pull out the addresses that are dragging your Microsoft reputation down. If you want help figuring that out, our SOS hotline is free and we'll look at your actual situation with you.