Outbound Glossary
Open rate
A number that used to mean something. Privacy proxies now load the tracking pixel for the recipient, so it mostly counts machines.
An open is recorded when a tiny invisible image loads. That worked while mail clients fetched images only when a person looked at the message.
Apple Mail Privacy Protection and similar proxies now fetch images in advance for everyone who has them on, so those opens happen whether or not a human ever saw the message. Treat open rate as a loose signal of deliverability at best, never as interest, and never report it to a client as engagement.