Glossary
Email open rate
The share of delivered emails recorded as opened, a metric increasingly distorted by privacy features that pre-fetch images.
Email open rate is the share of successfully delivered emails that were recorded as opened, calculated as opens / emails delivered. It is typically tracked with a tiny, invisible tracking pixel embedded in the email body: when the recipient's mail client loads that image, an open is logged.
That tracking method is also the metric's biggest weakness. Privacy features built into major mail clients, most notably Apple Mail's Mail Privacy Protection, pre-fetch images for many users regardless of whether the person actually opened or read the email, which can register an open that never really happened and makes open rate trend upward without any real change in engagement. This differs from email deliverability, which measures whether the email reached the inbox at all, a precondition for an open being possible.
Because of this measurement problem, many practitioners now treat open rate as a rough, increasingly unreliable signal and lean more on click-through rate and downstream conversion rate, which depend on an explicit user action rather than passive image loading. Open rate is still useful for relative comparisons, such as A/B testing two subject lines within the same send, where the pixel-inflation bias affects both variants roughly equally.
Last reviewed September 22, 2026