Mail's Inbox now fills the window with three columns: the folders, with unread counts and the mailbox's storage; the messages in the chosen folder, with search, stars and checkboxes for bulk actions; and the open message, with buttons to reply, reply all, forward, archive, delete, star and mark unread. New message writes in the third column. Opening a message or a folder no longer reloads the page.
On a narrower window the folders sit above the list. On a phone, the list and the open message take turns.
Mark unread now returns to the list, so the message stays unread.