Inbox placement varies sharply by ISP. A domain might deliver cleanly to Gmail and badly to Microsoft, or land in Gmail Promotions while hitting the Yahoo inbox. Diagnosing the difference requires per-ISP data, which means integrating with Gmail Postmaster, Microsoft SNDS, and Yahoo FBL. NimbusOS aggregates all three and surfaces them in a single ISP Monitoring dashboard. This article covers the three integrations, what each signal actually means, and the operational patterns for reading and acting on the data.
Why ISP Monitoring Matters
ISPs do not publish deliverability outcomes. They publish reputation signals that correlate with outcomes. Reading those signals is the closest a sender gets to knowing how their mail is actually being filtered.
Three ISPs are dominant in B2B cold outreach: Gmail (including Google Workspace), Microsoft (including Microsoft 365 Enterprise), and Yahoo. Together they represent well over 90 percent of typical B2B inbox coverage.
Each publishes different data. The ISP Monitoring dashboard shows them side by side so you can spot when one ISP is an outlier.
Gmail Postmaster Tools
Gmail Postmaster Tools is Google's sender-facing reputation dashboard. It shows per-domain and per-IP reputation, authentication pass rates, spam rate, and feedback loop identifiers.
What it reports
- Domain reputation. One of: High, Medium, Low, Bad. Computed over a rolling window.
- IP reputation. Same scale, per sending IP.
- Authentication. SPF, DKIM, DMARC pass rates.
- Encryption. TLS delivery rates.
- Delivery errors. Temporary (4xx) and permanent (5xx) error rates.
- Feedback loop. Complaint events from Gmail users.
Setup
Connect Gmail Postmaster in Integrations. The setup requires:
- Verify domain ownership with Google (TXT record).
- Create a service account in Google Cloud with the Postmaster Tools API scope.
- Upload the service account JSON to NimbusOS.
Once connected, the platform pulls data every 6 hours.
Reading the data
The reputation bands are the headline. Moving from Medium to Low is a warning; moving from Low to Bad is a crisis.
Spam rate is the actionable number. Gmail's threshold for triggering filter action is around 0.1 percent to 0.3 percent; above that, filtering intensifies.
Authentication pass rates should be at 100 percent. Any authentication failure visible here is fixable and urgent.
Microsoft SNDS (Smart Network Data Services)
SNDS is Microsoft's IP-level reputation dashboard. It is less detailed than Postmaster but covers the Microsoft ecosystem (Outlook.com, Hotmail, Microsoft 365).
What it reports
- Data volume. Messages sent through this IP to Microsoft.
- Complaint rate. User-reported spam.
- Trap hits. Messages delivered to spam trap addresses.
- Filter result. Green (good), Yellow (neutral), Red (bad).
Setup
Request IP whitelisting at sendersupport.olc.protection.outlook.com. Microsoft approves on a per-IP basis and can take several days. Once approved, SNDS delivers daily data.
NimbusOS pulls SNDS data daily and aggregates across all approved IPs in the fleet.
Reading the data
Filter result bands are the headline. Yellow is a warning to watch. Red is a crisis; delivery to Microsoft recipients will be heavily filtered.
Complaint rate is the actionable number. Microsoft's threshold for filter action is around 0.1 percent, same as Gmail.
Trap hits are rare but catastrophic. A single trap hit in a week can push an IP from green to red.
Yahoo Feedback Loop
Yahoo does not publish a reputation dashboard. It publishes a feedback loop: Yahoo users who mark mail as spam generate events sent back to the sender. NimbusOS ingests these events and aggregates them.
Setup
Register the sending domain with Yahoo's complaint feedback loop program. Yahoo requires DKIM to be signed and the complaint email address to be configured.
Once registered, complaints arrive as structured email messages to the configured address. NimbusOS parses them and writes to the feedback table.
Reading the data
Complaint rate per day. A healthy Yahoo complaint rate is under 0.1 percent; above 0.3 percent, delivery will be filtered.
Trend direction matters. A steady 0.08 percent is fine; a jump from 0.05 to 0.2 in one day is a warning.
The ISP Monitoring Dashboard
The dashboard at /deliverability/isp-monitoring has three tabs, one per ISP, plus an aggregate summary tab.
Aggregate tab
Rolls up the three ISPs into one health view. Useful for quick status. Shows:
- Gmail reputation band
- Microsoft filter band
- Yahoo complaint rate
- Aggregate inbox placement rate (weighted by send volume per ISP)
Gmail tab
Postmaster time series. Domain reputation, IP reputation, spam rate, authentication pass rates, TLS encryption. 30-day trend for each.
Microsoft tab
SNDS time series. Per-IP filter band, complaint rate, trap hits, data volume. 30-day trend.
Yahoo tab
FBL-derived complaint rate. Complaint counts per day. 30-day trend.
Cross-ISP Divergence Patterns
The dashboard is most useful when ISPs diverge. Common patterns:
Gmail good, Microsoft bad. Usually IP reputation, not content. Microsoft weights IP reputation more heavily. Check SNDS; consider IP warmup or rotation.
Gmail bad, Microsoft good. Usually domain reputation or content. Gmail weights domain reputation heavily. Check Gmail Postmaster spam rate and authentication.
All three bad. Usually a list hygiene problem. Run email verification on the active target segment. Check for recently added contacts from a low-quality source.
Gmail moved to Promotions not spam. The Postmaster spam rate will not show this because Promotions is not spam. Look at engagement rates. Pattern: normal open rate, low reply rate, mail in Promotions. Fix by reducing link count and removing marketing-looking language.
Alert Thresholds by ISP
The platform fires alerts when ISP signals cross thresholds.
Gmail reputation moves from Medium to Low. Warning alert. Not immediately actionable but signals degradation.
Gmail reputation moves from Low to Bad. Critical alert. Intervention required; consider pausing and investigating.
Gmail spam rate exceeds 0.3 percent. Warning alert.
Microsoft SNDS filter result Red on any IP. Critical alert. IP likely needs to be rotated or warmed separately.
Yahoo complaint rate exceeds 0.3 percent in 24 hours. Warning alert.
Alerts route through the standard deliverability alert system: in-platform, email, Slack, webhook.
Integration with the Deliverability Brain
ISP signals feed the NDS reputation component. A Gmail reputation downgrade drops the workspace NDS within 24 hours. The deliverability brain then adjusts campaign throttles and may pause cold outreach campaigns if NDS drops below the threshold.
The brain also uses ISP signals for domain rotation decisions. A domain with consistent Microsoft Red across multiple weeks is flagged for rotation.
What to Do When an ISP Alert Fires
A rough playbook.
Gmail Low or Bad. Pause new cold outreach campaigns from the affected domain or IP. Check spam rate; if high, review recent template content. Check authentication; fix any misalignment. Let warmup continue at reduced volume for 2 weeks before resuming cold outreach.
Microsoft Red. Pause sends from the affected IP. If the IP is dedicated to one inbox, pull the inbox from rotation. If the IP is shared, contact the SMTP provider about the neighborhood. Consider rotating to a backup IP while reputation rebuilds.
Yahoo complaint rate spike. Usually a specific campaign or template. Find the correlating campaign. Pause it. Review the target segment for unexpected additions. Rewrite the subject if trigger words are present.
Operational Limits
Three ISP-specific limits worth knowing.
Gmail recommends under 500 new recipients per day per domain for the first 30 days. NimbusOS warmup respects this.
Microsoft throttles new IPs aggressively. A freshly warmed IP can be throttled to 10-20 messages per minute per recipient domain until reputation develops.
Yahoo has stricter complaint rate enforcement than Gmail or Microsoft. A Yahoo complaint rate above 0.3 percent for even one day can degrade delivery for weeks.
Troubleshooting
"Gmail Postmaster shows no data"
Either the domain is too low volume (Postmaster requires a minimum send volume to publish data, around 100 messages per day to Gmail recipients) or the service account is not authorized. Check volume first, then re-authenticate.
"Microsoft SNDS shows no data even though I sent mail"
The IP is not whitelisted in SNDS. Request whitelisting through the Outlook sender support form. Approval takes up to 7 days.
"Yahoo FBL complaint address is not receiving events"
The DKIM signature may not be valid on your mail, which is required for Yahoo to send FBL events. Verify DKIM is passing on sends to Yahoo recipients.
Frequently Asked Questions
Can I monitor other ISPs (AOL, Proton, ProtonMail)?
Partial support. AOL is covered by the Yahoo FBL (Yahoo owns AOL). Proton and other niche providers do not publish public reputation data. The platform measures Proton through engagement rates and explicit bounce events but cannot monitor reputation directly.
Does monitoring require sending from my own domain?
Yes. Gmail Postmaster and Microsoft SNDS are both tied to domains or IPs you send from. A shared SMTP relay does not produce your own Postmaster data; it produces the relay operator's.
How fast do ISP signals update?
Gmail Postmaster publishes daily, with a 48 hour lag. Microsoft SNDS publishes daily. Yahoo FBL is near real time (events arrive within minutes of the complaint).
Can I correlate ISP reputation with specific campaigns?
Yes, in the Aggregate tab. Campaign-level delivery breakdown shows how much of each campaign went to each ISP, and the tab correlates campaign timing with ISP signal changes.
What to Read Next
Useful next pages after this one: Deliverability Overview for how ISP signals feed into the unified NDS, SPF, DKIM and DMARC for the authentication records ISPs require, and Email Warmup for how ISP signals drive the warmup stage engine.