If I were to guess, I'd say the same way adblockers work on browsers: blacklisting domains. Sendgrid was given as an example. They could also block using image size (1px) or location (end of message body).
That being said, I see your point. It's a game of Whack-a-Mole. What's to stop businesses from setting up a cname record on their domain to Sendgrid, for example?
That being said, I see your point. It's a game of Whack-a-Mole. What's to stop businesses from setting up a cname record on their domain to Sendgrid, for example?