Typical codes: R-OPEN-APPLE (Apple Mail loaded the image), R-OPEN-GOOGLE, R-OPEN-MSPROXY and R-OPEN-YAHOO (a provider’s image server), R-OPEN-GMAIL-PREFETCH and R-OPEN-GOOGLE-SCAN (Gmail fetched or checked it in advance), and R-OPEN-PREVIEW or R-OPEN-AI-DECLARED (a chat app or an AI assistant built a preview).
To check a message, open its event list and look at the category of each open. Events marked as a machine do not trigger notifications. An open marked unknown is only possible, and a click or reply tells you much more.