Deliverability metrics are dashboarded

Bounce rate, complaint rate, and open rate are the three signals ESPs use to determine whether your sending domain is trustworthy. When these metrics are not stored persistently and surfaced in a dashboard, deliverability collapse happens silently — Gmail and Yahoo start routing to spam, complaint rates climb past the 0.1% threshold, and you have no historical trend to show when you contact ESP support. CWE-391 (insufficient logging) at the operational layer means you cannot distinguish a transient spike from a structural reputation problem. Without queryable trend data, you are flying blind into potential domain blacklisting.

Create a persistent metrics table and write a query or dashboard panel that shows trend lines per signal. In your webhook handler (e.g., src/app/api/webhooks/email/route.ts), persist each event with enough context to aggregate later:

// On bounce webhook
await db.emailMetrics.create({
  data: {
    event: 'bounce',
    campaign_id: payload.campaignId,
    recipient_domain: domain,
    recorded_at: new Date()
  }
})

Query that table daily to produce bounce rate (bounces / sends) and complaint rate (complaints / sends) trend lines. Stdout logging does not qualify as persistent storage — the data must survive a process restart.