Runbook for deliverability drop

A deliverability drop — bounce rate spike, complaint rate above Gmail's 0.1% threshold, or inbox placement collapse — requires immediate, structured triage. Without a runbook, operators improvise: they may pause all sends instead of isolating the affected campaign, spend 30 minutes finding the right dashboard query, or miss the SPF/DKIM check that would have identified the root cause in 2 minutes. NIST SP 800-53 IR-8 (Incident Response Plan) requires documented procedures for anticipated failure modes. RFC 5321 bounce handling standards define the specific events this runbook must cover.

Create docs/runbooks/deliverability-incident.md with at least four sections: detection thresholds, triage checklist, immediate actions, and escalation contacts:

# Deliverability Incident Runbook
## Detection
- Bounce rate exceeds 5% over 1-hour window
- Complaint rate above 0.1% (Gmail threshold)
## Triage
1. Check bounce breakdown: `SELECT domain, count(*) FROM bounces WHERE ...`
2. Review Postmaster Tools for Gmail/Yahoo reputation
## Immediate Actions
- Pause affected campaign via admin API
- Verify DNS records (SPF, DKIM, DMARC)
## Escalation
- On-call: [name] via PagerDuty
- ESP support: [contact info]

The runbook must reference at least one numeric threshold — "bounce rate exceeds 5%" — to avoid the generic "contact ops" anti-pattern.