How does Apple Mail Privacy Protection (MPP) affect open rate reporting, and how should you adapt your strategy?

Apple Mail Privacy Protection (MPP), launched in September 2021, pre-fetches email content (including tracking pixels) via a proxy server when the email is delivered — not when the subscriber actually opens it. This means every email delivered to an Apple Mail user on iOS 15+, macOS Monterey+, or iPadOS 15+ registers as an "open" even if the subscriber never looks at it.
Impact: Open rates are artificially inflated (often 50–100% reported open rates for Apple Mail users). Open-based segments and A/B test winner criteria become unreliable.
Adaptations:
— Shift primary engagement metric to CTOR (click-to-open rate) or raw click rate, which are not affected by MPP.
— Use click-based engagement splits in Journey Builder instead of open-based splits.
— For deliverability monitoring, rely on click rates, unsubscribe rates, and complaint rates rather than open rates.
— Segment engaged subscribers based on clicks rather than opens for re-engagement and suppression logic.