I'm curious how they're doing it. Without having used their product, I gather that they're blocking certain known trackers and/or checking for tracking pixels directly. But it's not hard to put a unique tracking link on any arbitrary image which is part of the email's content. The only full defense is turning off all images.

edit: apparently they automatically cache the images on delivery, which should work. This is really a change that needs to come from the mail providers so good on them. As long as they don't try and assume what the user is interested in like Gmail does, they can drop tracking all they want.

That is going to lead to it appearing that people have high email engagement which will cause them to be on more mailing lists and at higher frequency than they otherwise would be.