Email Infrastructure Monitoring: Metrics That Actually Matter

Most email monitoring dashboards show vanity metrics. Here is what to actually watch - and how KumoMTA exposes all of it.

The Critical Metrics

Queue Depth

Number of messages waiting to be delivered. Growing queue = problem. This is your earliest warning signal.

alert: queue_depth > 10000
action: check reputation, throttle, investigate

Delivery Rate

Messages successfully delivered / messages attempted. Should stay above 98%. Spikes indicate reputation problems.

Bounce Rate by Enhanced Code

Not just total bounces - specific bounce types tell you what to fix:

• 5.1.1 (unknown user) spikes = list quality problem
• 4.2.0 (mailbox busy) spikes = temporary receiver issue
• 5.7.1 (policy blocked) spikes = content or reputation problem

Reputation Score

Google Postmaster Tools provides domain and IP reputation scores:

• Bad: Emails go to spam or rejected
• Poor: High risk of spam placement
• Medium: Normal scrutiny
• Healthy: Minimal filtering

KumoMTA Monitoring Endpoints

# Queue status
curl http://localhost:8000/api/queue

# Returns:
{
  "depth": 1247,
  "in_flight": 345,
  "connections": 45,
  "delivery_rate": "2340/minute",
  "bounce_rate": 0.023
}

# Per-domain stats
curl http://localhost:8000/api/domain/outbound.postmta.com

# Returns bounce classification
{
  "hard_bounce_rate": 0.008,
  "soft_bounce_rate": 0.015,
  "avg_latency_ms": 142,
  "gmail_complaint_rate": 0.001
}

Alerting Strategy

Set alerts at these thresholds:

• Queue depth > 5000: Warning
• Queue depth > 10000: Critical
• Bounce rate > 2%: Warning
• Bounce rate > 5%: Critical
• Complaint rate > 0.1%: Critical

The Dashboard Pattern

Build a real-time dashboard with:

• Queue depth (time series)
• Delivery rate (time series)
• Bounce breakdown (pie chart by enhanced code)
• Top bounce domains (bar chart)
• Reputation score (Google Postmaster API)

PostMTA provides this dashboard as part of managed service. For self-hosted KumoMTA, the Prometheus exporter built into KumoMTA connects to Grafana directly.

Log Analysis

KumoMTA logs every delivery attempt with full context:

2026-05-18T10:23:45Z delivery success to mx.google.com [192.168.1.1]
2026-05-18T10:23:46Z bounce 550 5.1.1 from mx.example.com [10.0.0.5]
2026-05-18T10:23:47Z retry scheduled for [e-mail] in 15m

Aggregate these logs to build your own BI reports on delivery performance over time.