Re: "The long term plan is to migrate our code to web technologies"
Well, if they deliver a "native" mail client using HTML/CSS/JS, it will be just a clumsy product with a lot of UI limitations.
Not really. This was an approach first mooted by Opera over 10 years ago but put on hold because of performance problems. Since then web components and JS runtime have improved beyond all recognition, which is why they're so prevalent on our phones. And if you're programming for a known runtime then you can avoid a lot of the compromises. This is the approach that the Vivaldi team (lots of ex-Opera people) have adopted and I'm looking forward to the promised mail client if it ever arrives.
As for e-mail itself: if only people would stick with text/plain then the world would be a simpler and safer place.