At enterprise cold email scale (100K+ emails per month), 10 to 20% of your active domains will burn every month. That's normal. The question is whether you detect it in time or find out two weeks later when your entire pipeline has dried up.
We manage 650K+ inboxes across Microsoft 365, Google Workspace, and SMTP for 1,500+ clients at MailDeck. Here are the five metrics we track per domain and the exact thresholds that trigger action.
1. Spam Complaint Rate
Where to check: Google Postmaster Tools, Microsoft SNDS
| Zone | Threshold | Action |
|---|---|---|
| Safe | < 0.1% | Continue sending |
| Warning | 0.1 - 0.3% | Reduce volume, review targeting |
| Pull domain | > 0.3% | Stop sending immediately |
Google's sender guidelines explicitly flag senders above 0.3%. At that point, the domain's reputation is damaged and recovery takes weeks of zero sending. Pull it at 0.3%. Waiting for 0.5% or 1% means you've already wasted hundreds of emails landing in spam.
2. Bounce Rate
Where to check: Your sequencer dashboard (Instantly, Smartlead, etc.)
| Zone | Threshold | Action |
|---|---|---|
| Safe | < 3% | Continue sending |
| Warning | 3 - 7% | Audit your list verification |
| Pull domain | > 7% | Stop sending, verify all remaining leads |
Prevention is the only real fix here. Verify every address through MillionVerifier or OmniVerifier before sending. At scale, even a 2% bad address rate compounds fast across thousands of emails per day.
3. Open Rate
Where to check: Your sequencer dashboard.
| Zone | Threshold | Action |
|---|---|---|
| Safe | > 30% | Continue sending |
| Warning | 10 - 30% | Investigate deliverability |
| Pull domain | < 10% for 7+ days | Domain reputation likely damaged |
One bad day is not a signal. A full week below 10% is. Note that Apple Mail Privacy Protection inflates open rates by preloading images, so treat this as a directional signal. A sudden drop from 45% to 8% is meaningful regardless.
4. Reply Rate
Where to check: Your sequencer dashboard.
| Zone | Threshold | Action |
|---|---|---|
| Healthy | > 4% | Copy and targeting are working |
| Investigate | 2 - 4% | Could be copy, targeting, or deliverability |
| Domain issue | < 2% on one domain while others are fine | Likely a deliverability problem with that specific domain |
4 to 5% is our baseline across clients. Reply rate is the hardest metric to act on because it blends multiple factors. Isolate by comparing the underperforming domain against your other domains running the same campaign.
5. Reputation Score
Where to check: Google Postmaster Tools, Microsoft SNDS
Google Postmaster rates reputation as High, Medium, Low, or Bad. Any domain hitting "Bad" should be pulled from sending immediately. Recovery from "Bad" is theoretically possible but takes weeks of zero sending, and some domains never fully recover.
Domain Rotation Math
Under active sending (3 to 5 cold emails per inbox per day), domains typically last 45 days to 2 months. Here's the reserve capacity you need:
| Active Domains | Monthly Burns (10-20%) | Reserve Needed |
|---|---|---|
| 10 | 1 - 2 | 2 - 3 ready |
| 30 | 3 - 6 | 5 - 8 ready |
| 50 | 5 - 10 | 8 - 12 ready |
| 100 | 10 - 20 | 15 - 25 ready |
Without warmed reserves, every burned domain means 7 to 10 days of lost capacity (purchase + DNS setup + 3 to 7 days warm-up). At 50+ active domains, you should be warming 5 to 12 new domains every month just to maintain capacity.
The Mistake That Cascades
Running identical templates across all domains. Same copy, same subject lines, same structure from 30 domains creates a fingerprint that email providers detect. When one domain gets flagged, the identical content fingerprint triggers flags on the others.
Fix: segment domains into independent groups. Each group gets its own template set, its own target audience segment, and its own sending schedule. Think of domain groups as independent businesses that share nothing detectable.
Quick Monitoring Setup
# Check domain reputation via command line
# SPF status
dig TXT yourdomain.com +short | grep "v=spf1"
# DMARC status
dig TXT _dmarc.yourdomain.com +short
# MX records exist
dig MX yourdomain.com +short
For automated daily monitoring across 50+ domains, set up alerts for:
- Spam rate crossing 0.1% (warning) or 0.3% (action)
- Bounce rate crossing 5% (warning) or 7% (action)
- Open rate dropping below 20% (warning) or 10% (action)
- Reputation change to Medium/Low (warning) or Bad (action)
All data based on Q1 2026 MailDeck platform metrics across 650K+ managed inboxes and 1,200+ domains.
Full operational guide with infrastructure planning calculator, all 6 mistakes that kill domain networks, and the complete monitoring stack: Read on maildeck.co
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