Open rate

Opens are counted by a tracking pixel loading when a message renders, and that mechanism has broken as a measure of human attention. Apple's Mail Privacy Protection, introduced in 2021, pre-fetches remote images for Apple Mail users whether or not anyone reads the email, so a large share of recorded opens correspond to nobody. Other clients do similar things.

The number still works as a relative measure: comparing subject lines on one list, or watching your own trend over months. Read as an absolute, or against a published industry average, it tells you almost nothing. Click rate and reply rate survive better.