I’m the author, nice to see this here! Happy to answer any questions. :)
I don’t recommend using HTML emails for newsletters as engagement goes way up if they don’t look like a newsletter.
Definitely understand this one.
Anyone remember NTK?
And Crypto-Gram is still going.
I think there’s a lot to be said for plain text, although sending plainish HTML as an alternative format. Designing an HTML email to look like an HTML web page is a turn-off. Rather than maintain two separate templates, you could either write your plain text in Markdown (etc) or write a fairly minimal HTML template and convert it back …