Microsoft SNDS monitor guide for Outlook sender reputation
Use Microsoft SNDS to interpret Outlook complaint, trap-hit, filter, HELO, and IP reputation evidence before sender issues become blocks.
Microsoft's Smart Network Data Services (SNDS) is the only window most senders ever get into Outlook.com filtering. It is free, frequently updated, and almost always misread — usually because the column names are terse and the documentation is spread across three different help portals. Pair it with domain monitoring, deliverability testing, and the API docs if you need repeatable checks around Outlook reputation.
What each SNDS column means
| Column | What it shows | Threshold to worry |
|---|---|---|
| RCPT commands | Total recipients attempted | Context for ratios |
| Data commands | Messages actually transferred | Should match RCPT |
| Message recipients | Unique recipients reached | — |
| Filter result | GREEN/YELLOW/RED bucket | Anything but GREEN |
| Complaint rate | % of recipients who hit 'Junk' | Above 0.1% |
| Trap message period | Window when traps were hit | Any non-empty value |
| Trap hits | Spam-trap addresses contacted | 1 or more |
| Sample HELO | EHLO/HELO string used | Should match rDNS |
| Sample MAIL FROM | Return-path used | Should be on a domain you control |
The three SNDS warning signs
- Filter result flips to YELLOW. You have ~72 hours before delivery degrades. Pause campaign volume and identify the offending segment.
- Trap hits appear for the first time. A trap is an address that should never receive mail — hitting one means your acquisition or hygiene process is broken.
- HELO/MAIL FROM mismatch with rDNS. Microsoft penalizes infrastructure that does not reverse-resolve cleanly to its sending identity.
What to do when SNDS turns yellow
| Signal | Likely cause | First action |
|---|---|---|
| Complaint rate rises | Wrong segment, stale list, or unclear unsubscribe | Pause the campaign and validate the next send list |
| Trap hits appear | Purchased, scraped, or badly aged addresses | Suppress the source list and review acquisition |
| Filter result is YELLOW | Microsoft is seeing a pattern, not a one-off | Reduce volume, test the template, and document the fix |
Use SNDS as a trigger, not as the whole diagnosis. A YELLOW day should send the team to the bulk sender checklist, a fresh deliverability test, and list validation before the next Outlook-heavy campaign.
Pairing SNDS with JMRP
SNDS gives you the rate; JMRP (the Junk Mail Reporting Program) gives you the addresses. Sign up at the same portal, register a complaint-handling mailbox, and Microsoft will forward every 'Junk' click. Suppress those addresses immediately — continuing to mail them after a complaint is the fastest way to a permanent block.
Frequently asked questions
| SNDS signal | What it suggests | Supporting evidence |
|---|---|---|
| Filter result YELLOW or RED | Outlook is seeing a sender pattern worth slowing down | Run a deliverability test and compare recent list/source changes |
| Trap hits | Bad acquisition, old list, scraped data, or poor suppression | Validate the next list and suppress risky segments |
| Complaint rate increase | Expectation mismatch, stale audience, or weak unsubscribe path | Inspect List-Unsubscribe and segment engagement |
| Sample HELO mismatch | Infrastructure identity problem | Check PTR/rDNS, HELO, and domain monitoring evidence |
Pair SNDS with domain monitoring, DMARC monitoring, the email deliverability tester, and email validation before resuming Outlook-heavy volume.
Continue this inbox placement and reputation monitoring workflow with the commercial page, the core guide, the implementation docs.
Last updated June 13, 2026.
Sources reviewed
- Microsoft Smart Network Data Services(official)
Factual review: June 13, 2026 by WillItInbox Editorial.
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