Open rate is the share of delivered emails that were opened, and since privacy protections became widespread it is a directional signal rather than a precise one.
Opens are counted by a tracking pixel loading. Mail privacy features now pre-load that pixel for many recipients whether or not they read anything, which inflates the number and makes cross-period comparisons unreliable if the audience mix changed.
It remains useful for comparing subject lines within the same list and period, where the inflation applies equally to both. It is no longer useful as an absolute benchmark.
Click rate and reply rate survived the change better, because they still require a deliberate action.
In practice
A subject line test showing 41% against 33% within the same send is meaningful; the same 41% compared against last year's is not.
Common mistake
Reporting open rate to a sponsor as audience engagement. Anyone informed will ask about click rate instead.
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