Effective IP warmup requires active monitoring — not set-and-forget. ISP signals change daily during warmup, and catching a problem early allows you to address it before it sets back weeks of progress. This article covers every monitoring tool and metric you should track throughout the warmup process, with specific guidance on interpreting the data.
The Warmup Monitoring Stack
Use all five of these in combination — no single tool gives the complete picture:
| Tool |
What It Shows |
Check Frequency During Warmup |
| MigoSMTP Delivery Reports |
Delivery rate, bounce rate, complaint rate, open rate, queue status — for every send from your account |
After every send; daily summary |
| Google Postmaster Tools |
Gmail's direct view of your domain and IP reputation; spam rate as seen by Gmail; authentication pass rates |
Daily — especially in weeks 1–2 |
| Microsoft SNDS / JMRP |
Outlook/Hotmail's view of your IP reputation and complaint data from Outlook users |
Every 2–3 days |
| Inbox placement tests |
Real-world check of whether your emails are landing in inbox vs spam across multiple email providers |
Twice per week during active warmup |
| MXToolbox Blacklist Check |
Checks your domain and IP against 100+ industry blacklists simultaneously |
Weekly, or immediately if delivery rate drops |
Google Postmaster Tools — Setup and Interpretation
Setup (Do This Before Warmup Begins)
- Go to postmaster.google.com.
- Click Add Domain.
- Enter your sending domain (e.g.
mail.yourcompany.com).
- Verify ownership by adding the displayed TXT record to your DNS.
- Once verified, data for your domain begins populating after your first Gmail deliveries (minimum ~200 messages to Gmail addresses needed for data to appear).
Key Postmaster Tools Metrics and What They Mean
| Metric |
What It Measures |
Target During Warmup |
| Domain Reputation |
Gmail's overall assessment of your sending domain: Bad / Low / Medium / High |
Start at Low/Medium; should reach High by end of month 1 |
| IP Reputation |
Gmail's assessment of the specific IP(s) used to send your email |
Progresses from Low → Medium → High as warmup progresses |
| Spam Rate |
% of your emails that Gmail users marked as spam, as seen by Gmail |
Must stay below 0.10%; alert if it exceeds 0.08% |
| Authentication |
% of your emails passing SPF, DKIM, and DMARC authentication checks |
Must be 100% for SPF and DKIM; investigate immediately if below 99% |
| Delivery Errors |
Volume and type of Gmail delivery errors (5xx vs 4xx, rejection reason) |
Zero 5xx errors; some 4xx deferral is normal early in warmup |
Interpreting Warmup Progress Signals
| Signal |
Interpretation |
Action |
| Domain reputation improving from Low → Medium |
Warmup is progressing normally |
Continue schedule as planned |
| Open rate >35% consistently |
Recipients are engaging — positive reputation signal to ISPs |
This is good; continue sending to engaged segments |
| Domain stuck at Low after 7 days |
Not enough volume or engagement to move the needle, OR complaint signals preventing progression |
Check spam rate in Postmaster Tools; increase send frequency (daily not every 3 days) |
| High deferral rate to one ISP |
That ISP is throttling your new IP — normal but needs monitoring |
Do not skip the throttling; let the retry queue work through it; reduce daily volume to that ISP temporarily |
| Spam rate rising in Postmaster Tools |
Recipients are marking as spam at a rate that will damage your reputation |
Pause; review content and consent; switch to a higher-engagement segment; do not increase volume |
| Domain reputation drops to Bad |
Critical failure — Gmail is blocking most of your email |
Stop all sending; full audit required; contact MigoSMTP support; consider whether this domain can be recovered |
Inbox Placement Testing
Inbox placement tests tell you directly whether your emails are landing in the inbox, the spam folder, or being blocked — at specific email providers. Use these tools twice per week during warmup:
| Tool |
How to Use |
Free? |
| GlockApps |
Send to their seed list → see inbox/spam results per provider within minutes |
Free tier available (limited tests/month) |
| Mail-tester.com |
Send to a temporary address → receive spam score and basic placement feedback |
Yes — 3 free tests per day |
| Litmus Spam Testing |
Tests through multiple spam filters and shows inbox placement across providers |
Paid (included in Litmus subscription) |
| Manual seed list |
Create test accounts at Gmail, Outlook, Yahoo, and Apple Mail; send to them and check directly |
Free — requires setup time |
Warmup Monitoring Calendar
Suggested daily monitoring tasks during active warmup (weeks 1–4):
| Frequency |
Task |
| After every send |
Check MigoSMTP Delivery Reports for bounce rate and complaint events via webhook |
| Daily |
Review Google Postmaster Tools domain reputation trend; check spam rate |
| Every 2–3 days |
Run inbox placement test via GlockApps or manual seed accounts |
| Weekly |
Run MXToolbox blacklist check; review Microsoft SNDS data; assess whether to advance to next volume stage |
| Before each volume increase |
Confirm all metrics at the current stage meet the pass criteria before stepping up |
Next Steps