A tracker that shows only “opened” or “not opened” cannot tell these cases apart. HawkOpen checks each open and click against named rules, for example Apple Mail Privacy Protection, Gmail and Microsoft image proxies, security scanners and burst clicks, and shows the label and rule code. The result is still an estimate, at most “likely”.
The most reliable signals are a reply and a click that is not from a scanner. If two tools track the same email, each adds its own tracking image, and both are affected by the same limits.