Email Deliverability Metrics

Track these metrics to maintain consistent inbox placement

The Primary Metric: Inbox Rate

Inbox rate is the percentage of emails delivered to the inbox versus spam folder. This is your north star metric.

97%+
Target Inbox Rate
Measured across all major ISPs. If you're below 97%, investigate immediately.

How to Measure Inbox Rate

  • Seed list testing – Send to known inbox and spam seed accounts at each ISP
  • Postmaster tools – Gmail, Outlook, Yahoo postmasters show spam vs inbox
  • Campaign analytics – If open rates drop suddenly, inbox placement likely changed

Core Deliverability Metrics

1. Delivery Rate

98%+
Target Delivery Rate
Emails that reach the recipient's mail server (inbox or spam). Calculated as: (Emails Delivered / Emails Sent) × 100
Target: Above 98% delivery rate. Below 95% indicates infrastructure or list problems.

2. Bounce Rate by Type

Bounce Type Alert Threshold Action Threshold Action
Hard Bounce 0.5% 1% Remove immediately
Soft Bounce 2% 5% Retry 3 times
Block Bounce 0.1% 0.3% Stop sending, investigate

3. Complaint Rate

<0.1%
Target Complaint Rate
Percentage of recipients marking as spam. Gmail and Yahoo are most sensitive.

⚠️ Complaints Are Reputation Killers

A single complaint can trigger ISP filters. If complaint rate exceeds 0.1% at Gmail, your entire sending domain risks being flagged. Set up FBL (Feedback Loop) integration to monitor in real-time.

Reputation Metrics

4. IP Reputation Score

Each sending IP has a reputation score maintained by ISPS and third parties:

Score Range Reputation Expected Deliverability
90-100 Excellent Full inbox placement
70-89 Good Normal inbox placement
50-69 Neutral Some filtering possible
25-49 Poor Heavy filtering expected
0-24 Bad Blocklist candidate

5. Domain Reputation

Domain reputation is tracked separately from IP reputation. Gmail especially focuses on sending domain.

  • New domain – Starts neutral, builds reputation with consistent sending
  • Branded domain – If you send consistently, reputation compounds
  • Burned domain – Reputation damage takes months to recover

Engagement Metrics

Modern ISP filtering heavily weights engagement signals:

20%+
Target Open Rate
For cold email. Opens indicate interest and prevent spam folder routing.
5%+
Target Reply Rate
Replies are the strongest positive signal. Even auto-replies count toward this metric.

Reply Rate is the Key Metric

Research shows that emails with 15%+ reply rates maintain 95%+ inbox placement even during reputation fluctuations. The reply signal tells ISPs: "This is a real conversation, not spam."

6. Spam Folder Rate

Target: Below 2%. Higher rates indicate reputation or content problems.

If more than 2% of your delivered emails land in spam, investigate:

  • Sender reputation drop
  • Content triggering filters
  • Sudden volume increase
  • Authentication failures

ISP-Specific Metrics

Track metrics per ISP since each has different thresholds:

ISP Key Metric Alert Threshold Monitor Tool
Gmail Bounce rate 0.3% Gmail Postmaster
Outlook Junk rate 3% SNDS (Smart Network Data)
Yahoo Complaint rate 0.08% Yahoo Postmaster
iCloud Block rate 2% Manual testing

Daily Monitoring Dashboard

Build a monitoring dashboard tracking these metrics:

Minimum Daily Check

  • Total volume sent vs previous day
  • Delivery rate by ISP
  • Hard bounce rate
  • Complaint rate
  • IP reputation scores

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