Things like Mailtrack are against privacy, aren’t they? Especially because it’s not in the control of receivers at all. Or am I missing something?!

When they go as far as logging when and how many times any by which client (desktop app / browser) it was opened, that's just being a stalker.

When it's as far as "received on x" that's not overly bad.

But the receiver should be notified if there's something watching, and the shitty pixel trackers should be removed.

Its an addon that is embedding a tracking pixel. Nothing new or innovative.
if you mean against privacy against the gdpr, i can say yes.
The reciepient didn't consent to tracking,when you use it for the first email.
And why does anyone need tracking for emailreading? email is an async communication.

Yes, it's an invasion of privacy. You can't really opt-out of it currently.

There's a few ways to do it and some clients even has built-in tools for it. Often it's done by adding an image to an email which when is loaded by the email client is logged by a backend. They're abusing and circumventing how emails are supposed to work.

It's something that is pushed by people who do marketing so they got more metrics. However there's so many invasive things they currently do. Which is why I love stuff like the Brave browser which has built-in functionality block or even feed websites fake data to prevent fingerprinting (which is a currently a serious issue and super easy to do) and behaviour analysis on the fingerprint across the web.

Pretty much ever email service allows this. The pixel that is tracked can be a third party one such as Google or Facebook and is just part of the email. The email service usually doesn't care about the content that is sent to the user.

I work in a marketing analytics company that puts out those emails campaigns.