make Outlook look exactly like Thunderbird
Here is the main problem: The header in the preview pane is ridiculously big. There are whole strings complaining about this online going back three or four years.
But even with that, the online web version is completely not doable for me. I cannot use a system that is so poorly designed. The message list is the worst.
I am trying to work with the Outlook app, which is not that great. With the app, I am working with various views and customizing them. If MS would deal with the header and the ridiculous little pop-up notes, the attachments, and so on... if you get that stuff out of there, so that it can be a three-line header as in Thunderbird, you might get more positive reviews and happy end-users.