Engagement scoring uses multi-signal approach (not opens alone)

Apple Mail Privacy Protection (MPP), launched in iOS 15, pre-fetches email content through a proxy — generating open events for emails that were never read by a human. Systems that branch or score exclusively on open events are routing contacts based on machine-generated signals, not human intent. A contact's sequence may advance to the "engaged" branch because Apple's privacy proxy opened the email — no human action required. iso-25010:2011 functional-suitability.functional-correctness classifies this as a functional accuracy defect: the system computes engagement from unreliable inputs, producing systematically incorrect routing for a large portion of Apple Mail users.

Weight engagement signals by reliability: replies and bookings are definitive human actions; clicks are reliable; opens require MPP adjustment before use in scoring or branching decisions.

const ENGAGEMENT_WEIGHTS = {
  reply:         10,  // Definitive — human authored
  booking:       15,  // Highest-intent signal
  click:          5,  // Reliable — human action
  website_visit:  3,  // Reliable if UTM-tracked
  open:           1,  // Unreliable — MPP generates false positives
} as const

function computeEngagementScore(events: EngagementEvent[]): number {
  return events.reduce((sum, e) => sum + (ENGAGEMENT_WEIGHTS[e.type] ?? 0), 0)
}

For branching decisions, use clicks or replies as the threshold trigger — not opens alone. If opens must be used, apply a suppression window: ignore opens that arrive within 5 seconds of delivery (a proxy fingerprint) or that occur without any subsequent click.