Open rate
The percentage of email recipients who opened the email — historically the headline engagement metric, now corrupted by Apple Mail Privacy Protection.
Open rate is the percentage of email recipients who opened the email, calculated as (unique opens / sends) × 100. Historically the headline engagement metric. Since 2021, Apple Mail Privacy Protection (MPP) has preemptively loaded images for Apple Mail users — which trips the open-tracking pixel whether or not the user actually opened the email.
For audiences skewing heavily toward Apple Mail (consumer DTC, lifestyle brands, B2B with iPhone-heavy users), open rates are now significantly inflated. Industry benchmarks suggest open rates have effectively become "rate at which the email arrived in any folder," not "rate at which the recipient actually opened."
Modern email programs are shifting primary engagement measurement to click-through rate, which doesn't suffer from MPP inflation. Open rate still has uses — subject-line A/B tests, comparative analysis within the same audience over time — but should no longer be treated as the headline metric. Reporting open rate without acknowledging MPP is increasingly considered poor analytical practice.