/ Glossary — Metrics
What is open rate?
The share of delivered mail recipients open — once a core engagement metric, now distorted by privacy features that inflate it. Here's what it still tells you and what it no longer does.
/ Definition
Open rate is the percentage of delivered emails that recipients open. Traditionally it was measured with a tiny invisible tracking pixel that loads when the message is viewed, and it served as a primary gauge of how engaging your subject lines and audience were. It remains one of the most-watched email metrics, even as its reliability has eroded.
The complication is Mail Privacy Protection. Since Apple introduced it, Mail privately pre-loads images — including the tracking pixel — for many users whether or not they actually open the message, which inflates open rates and makes them an unreliable measure of real engagement. Open rate is now best read as a rough, upward-biased signal rather than a precise number.
/ In practice
Why open rate became unreliable
The tracking pixel that measures opens depends on images loading when a human views the message. Apple's Mail Privacy Protection broke that assumption by pre-loading images on Apple's servers for a large share of users, regardless of whether the message was opened — so those users register as opens whether they read the mail or not. Because Apple Mail represents a substantial portion of email clients, this inflates aggregate open rates significantly and unpredictably, which means comparing open rates across time or audiences is far less meaningful than it used to be.
What to use instead
With open rate compromised, engagement is better judged by signals that reflect genuine action: clicks, replies, conversions, and the absence of complaints or unsubscribes. For reputation purposes, the metrics that actually move deliverability are bounce rate, complaint rate, and real engagement — not opens. Open rate still has some diagnostic value for spotting large directional changes, but treating it as a precise engagement number, or optimizing subject lines purely against it, is building on a distorted foundation.
/ Open rate after the privacy shift
How to use it responsibly now
The practical stance on open rate today is to treat it as directional, not precise. A large drop or jump can still flag something worth investigating, and relative comparisons within a single audience over a short window retain some meaning. But using open rate to compare audiences, to measure absolute engagement, or as the primary success metric for a campaign now builds on a distorted foundation, because the Mail Privacy Protection inflation isn't uniform — it varies with how many of your recipients use Apple Mail. The senders who adapted fastest simply moved their optimization to clicks and conversions, metrics that reflect a real human action rather than an automated image fetch, and kept open rate only as a rough secondary signal.
/ Related terms
Open rate tells you less than it used to. We track what still matters.
We monitor the engagement and reputation signals that actually move deliverability, not the ones privacy features have distorted. Bring us your sending and we'll measure what counts.
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