Comment by kkfx

Simple meaning: I'm not tied to anyone specifically (personal domain name) and I own my data. They are also on someone else iron, but also on mine and I use them locally. Composing a new mail is just hitting a key on my keyboard, searching my messages like GMail is another key for search&narrow results a modifier+the same key for notmuch-emacs UI. All mails can be linked on all my org-mode/org-roam managed docs equally.

It's FAR simpler and FAR more powerful than any modern crapware UI, BUT is hard to setup due to the little development compared to the mainstream UI.