Tracking pixels are ubiquitous, and used in virtually almost every situation where you want to know if a person opened your email, which equates to virtually all ecommerce in my experience.

As you said, in newsletter/spam business. But common mail apps don't do that - there is already a defacto standard for that. It's clear that Nylas business is around tracking and collecting data. It's like Win10, the user is the product and everything gets collected.

Good that Outlook and other mail apps blocks third party pictures by default.

There are other mail clients that use tracking pixels, like Polymail and Canary.

The parent mentioned probably the two biggest mail clients in the world, and you mentioned 2 that I haven't heard of despite 20 years of sending email and toying around with new software.. I'm not sure it's an effective rebuttal.