What Are Email Open Rates?
Open rates are a measure of what percent of users opened your email.
Open rates can be calculated on a unique and non-unique basis.
Unique open rates only count one open for each recipient. Non-unique open rates count all opens regardless if a user opens the same email multiple times.
Open rates are tracked by adding an invisible tracking pixel to emails. When a user opens the email the tracking pixel fires and tells the email sending service that it was opened.
The latest changes to iOS14 gave users the ability to prevent those tracking pixels from firing, which skews open rates when looking at statistics for devices running that mobile operating system version.