Open rate

Open tracking adds a tiny, invisible image to your email. When the recipient's email app loads the image, the sender records an open. The open rate is the number of opens divided by delivered emails.

This method has many gaps. Apple Mail Privacy Protection loads images for its users in the background, so an email can count as opened when no one read it. Some providers fetch images through a proxy or cache, and security scanners open links and images too. At the same time, people who block images are never counted. The result can be too high or too low, and you cannot tell which.

Treat open rate as a rough hint at best. Use it to compare two subject lines inside the same campaign, never as proof that people read your mail. Replies and booked meetings are the numbers to follow. Some senders turn open tracking off, because the tracking image and the extra domain it uses can also count against you with spam filters.