Unless your recipients use email clients that don't load remote http images except on demand.

I can think of one email client with ~15 million users that does that.

That's not a good excuse for not implementing a standard. It's also a little like saying "private investigators are still the most reliable way to know whether a person is at home": it's true, but not really a better thing to implement.

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.

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.

It's actually hard to track down exactly when these images are cached. I don't really trust MailChimp to be truthful about this but I also can't find any specific from Google themselves about whether that caching happens when their SMTP receives the email or when the user actually opens the message.

Since Google's goal with this was clearly to defeat tracking, I would strongly expect the former, but I can't back this up.