Litmus' 2017 State of Email Deliverability report touches on verification services:
Our research found that BriteVerify and FreshAddress were the most popular providers by far.
Thanks, Richard. That's exactly how I've heard others using the 1.5X image strategy--when they're concerned about the overall loadtime of an email. If you have an email with only one or two large images, going 2X (or even 3X) isn't really an issue. It's when you have many images in an email or animation, as Richard points out, that compromises on image resolution start to make sense.
The iPhone X has a tiny market share currently, so I don't know that anyone should be rushing to move to 3X. However, the momentum toward high-DPI screens is clearly gaining steam. I know that because of very real load-time considerations that some brands use 1.5X images in their emails. Perhaps the takeaway here is that more brands should be moving all the way to 2X this year.
Luke, I think you're right. These are more accurately called "HTML-lite" emails, since they surely include tracking pixels if nothing else. But, yes, the goal is (1) quicker load time and (2) more casual or urgent feel.
I personally think marketers should be looking to use these very selectively. HTML emails with plenty of images are the norm in many verticals--and for good reason. HTML emails perform better. But there are exceptions. B2B brands have some of the more compelling use cases. And triggered emails are one place where "HTML-lite" emails can make sense.