See my peer comment: Apple Mail really does download external images automatically, on a server to hide your IP address. It then rewrites those emails to reference the cached images from the server. Every tracker pixel sent to a mail account with this feature will be counted as read, whether or not the user opens it.
Ah I didn't see that one, they mention this on another page [0] (scroll a bit down). But they mention also that it still reliably detects views and that google released a new feature a while ago "dynamic email", which automatically loads but can also be disabled.
Maybe my knowledge about Mail clients is a bit skewed, because I'm using open source Mail clients on every platform (Thunderbird, K-9 Mail, Evolution), and they always block External content by default. But I thought this was the standard today, but I guess loading them on the server is a bit more UX friendly instead of blocking remote content, although I wouldn't want it.
