Spying on people is extremely easy, still that does not make it morally correct
I just crossed an article about Superhuman1 in which you can see that the e-mails sent through that system all include a "tracking pixel" which is just a tool to spy on people who receive the e-mail. Particularly, Superhuman shares the information with the senders of the messages. You can see how often someone opens your e-mail, and their location.
E-mails, in my perspective, are the most intimate aspect of the internet. Is the tool that we are completely free to choose. The fact that it is so easy for someone with or without a complex service to know what client I use, how often I open e-mails, where I am located, made me wonder how vulnerable is our privacy.
The bottom question, always, is whether you should do something simply because you can do it. Many people don't stop to think about it, and what is more worrisome is that even people with decision making abilities, such as the developers at Superhuman, their investors, their users, don't get to rationally arrive to the conclusion of avoiding these practices.
And it is not a matter of getting statistics such as the opening rate of a newsletter, it is really getting individualized information about a single person. That e-mail you sent to an ex, you can still check whether they open it, and where they are.