Open rate

Open tracking works by adding a tiny invisible image to each email; when the image is loaded, the email counts as opened. Several things now break that. Some mail apps load images automatically to protect privacy, so opens are recorded for emails nobody read, while others block images, so real reads go uncounted. Corporate security filters may also fetch images while scanning a message.

The tracking itself has a cost in outbound: the image and the rewritten links it often comes with make an email look more like bulk mail to spam filters. For these reasons many outbound teams switch open tracking off and judge campaigns on replies, positive replies and meetings instead. Opens are still a rough sign that something is broken, such as a sudden fall to near zero after a domain change, but not a basis for choosing between two emails.