Not only that, but if your recipient has an iPhone and uses the Mail app (and a few other platforms now), the images are all loaded on Apple (or other) servers

Not only that, but if your recipient has an iPhone and uses the Mail app (and a few other platforms now), the images are all loaded on Apple (or other) servers first as part of a privacy and safety process, registering as an Open. So it's even harder to tell if people are actually opening the emails or not.