I was prepared for disappointment, given the sad direction most desktop applications have been heading in the past decade. But admittedly, it looks pretty dang good. Looking forward to giving it a test drive!
It hit me that they may be listening to their users more than most desktop app developers when I scrolled to the section about display density. The overwhelming majority of modern desktop apps have exploded the use of white-space and grossly reduced the information density of applications. Mind you, I suspect I will personally want an even more dense setting than their current densest option. But having density settings demonstrated front and center gives me hope.
I'm also concerned that I will still want a traditional three-pane split view (folders at left, message list at top-right, message text at bottom-right). Many desktop users do not run their mail client fully maximized, so a horizontally-biased window size should not be assumed. I run Thunderbird on the left half of a single monitor, meaning it's taller than it is wide. A three-column view will probably not use that space well.
I'm guessing this is not an Electron app. If I am right, that alone deserves praise.
In sum, I am now cautiously optimistic!