What exactly counts as an open?
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An "open" is recorded when a tracking pixel, a tiny invisible 1×1 image embedded in the email, loads. When that image loads, your ESP registers the time, IP address, and user agent, and marks the message as opened.
The problem: image loading doesn't always mean a human opened the email. Three scenarios inflate open rates significantly.
Apple Mail Privacy Protection (MPP), launched in iOS 15, is the biggest one. Apple Mail preloads remote content for all messages before the recipient sees them. Effectively marking every email opened by an Apple Mail user as opened, even if they never actually viewed it. For senders with significant iOS audiences, this inflates open rates by 20–40% or more depending on audience mix.
Mail prefetching by security tools. Corporate email security gateways and spam filters often load images automatically to scan for malicious content before delivery. That scan registers as an open.
Bot traffic and proxy servers. Similar to security scanning. Automated systems loading email content before or instead of the human recipient.
Practically, this means open rate as an absolute number is unreliable for many senders. What's still useful: open rate trends over time within the same audience (relative changes still signal something), and open rates as a comparison between segments. Click rate is a more reliable engagement proxy because clicks require deliberate human action.
If you're using opens to define your engaged subscriber segments, switch to click-based definitions. Especially if you have a large Apple Mail audience.
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