Did you know that, by default, your email sends information to mailing list platforms about your reading activity? The platform gets to know if you opened the message, and often how far along you've read in it.
What is this shitty email program they're talking about? Sure, they can embed a 1-pixel tracking image to see when you opened the email (if you allow auto-loading images), but how would they know how much you've read unless some incredibly horrible email program actively sends out that data?