First opens are reliable when the recipient's client loads images. Beyond that there
are limits worth knowing. Spam scanners, link-preview bots and Apple Mail Privacy
Protection all fetch the pixel without a human reading anything, so we classify and
exclude those and show you every filtered event with the reason.
Repeat open counts are a lower bound rather than an exact number, because Gmail caches
images through its own proxy. And if a recipient blocks images, no receipt exists at
all, which means no information rather than an unread email.