Post-MPP (iOS 15+), Apple Mail registers near-100% opens via image prefetch — campaign-level open rate is now mostly a list-composition signal.
Open rate is computed over the same campaign or flow. “Unique” caps each recipient at one open in the numerator; most ESPs also expose a total-opens variant that counts every pixel load, which is why the same campaign can show two different open numbers in the same dashboard. The mechanic underneath is a tracking pixel — a 1×1 image embedded in the email HTML whose load on the recipient’s client registers the open. No pixel load, no recorded open, even if the recipient read every word.
Apple’s Mail Privacy Protection (MPP), which shipped in iOS 15 in September 2021, broke that mechanic at consumer scale. For Apple Mail users who opted in at first launch — which is most of them — Apple pre-fetches images, including tracking pixels, on its proxy servers regardless of whether the recipient ever opens the message. The result is that opted-in Apple Mail users register near-100% open rates as a class, on Apple’s prefetch timing rather than the recipient’s actual open time. Apple Mail is a large share of the consumer email base; Litmus’s quarterly email-client reads have placed Apple Mail and iPhone Mail combined in the 50–60% range at consumer-focused senders in the post-MPP period, though the exact split varies by list composition. The same privacy-era pattern shows up on the ad side as ATT.
What the number now means has shifted. A campaign-level open rate is mostly a signal of list composition — how much of your list is Apple Mail — rather than actual engagement. Operators have adjusted in three ways. Where the ESP exposes a mail-client field, they segment opens by client and read Apple opens as delivery confirmation (the message reached an inbox a pixel-prefetcher could see) rather than engagement. They move engagement reporting onto CTR (clicks / delivered) or CTOR (clicks / opens), both of which require an actual click. And they rewrite open-based “engaged segment” filters — “opened in the last 30 days” without MPP-aware logic keeps Apple subscribers perpetually engaged because the prefetch fires on every send, while non-Apple subscribers age out after a few quiet weeks. The right segment definition pairs opens with a client check or leans on clicks instead. The corresponding deliverability read is covered in email deliverability.
Open rate is not dead. It is a different signal than it used to be — a read on who is on your list, not on whether they read the message.