When emails aren't reaching inboxes, Postmaster Tools and SNDS are the first places to look — they show you exactly what Gmail and Outlook think of your sending.
Gmail Postmaster Tools
Setup
- Go to postmaster.google.com
- Add your sending domain
- Add the provided TXT record to DNS to verify ownership
Key Metrics to Monitor
- Domain Reputation: Bad / Low / Medium / High. High is your goal. Anything below Medium means filtering.
- Spam Rate: Percentage of your emails marked as spam by Gmail users. Keep below 0.1%. Above 0.3% = significant filtering.
- Authentication: SPF, DKIM, DMARC pass rates. All should be 100%.
- Delivery Errors: Specific SMTP error codes that explain rejections.
- IP Reputation: If different from domain reputation, your sending IP may have issues.
Interpreting Results
If spam rate is high but you're sending to opted-in subscribers, check: are you sending too frequently? Is your content irrelevant? Are old subscribers still on the list?
If authentication fails at less than 100%, you have a configuration problem — specific sending source not covered by your SPF/DKIM setup.
Microsoft SNDS (Smart Network Data Services)
Setup
Visit sendersupport.olc.protection.outlook.com/snds/. Register and add your sending IPs.
What SNDS Shows
- Trap Hits: Number of spam trap addresses you're hitting. Any non-zero value needs immediate investigation.
- Complaint Rate: Green (clean), Yellow (caution), Red (take action)
- Filter Verdict: OK / Bulk / Filtered — shows how Outlook is classifying your email
Action Plan by Status
- Domain: Bad + Spam rate high: Stop sending, deep clean list, restart warmup
- Domain: Low: Improve engagement, remove inactives, send more relevant content
- Auth failures: Fix SPF/DKIM for the specific source that's failing
Fix reputation with proper warmup — ZeroPhantom Email Warmup →