* Vertical View is the default.
* Toolbars are consolidated into one "Mail Toolbar"
* Mail Toolbar can be toggled between visible and hidden.
** Mail Toolbar has three flexible sections that re-size with their respective Server/Folder panel, Thread panel, and Message panel.
** Toolbar customization is section specific.
*** Server/Folder panel's toolbar includes "Get Messages" and "Write" by default. "Address Book" is optional.
*** The Search & Filter toolbar above the Thread panel has all items included by default.
*** The Message panel's toolbar has "Reply", "Multi-reply", "Forward","Archive", "Junk", and "Delete" by default. "Tag", "Print", and "Save" are optional.
** Mail Toolbar buttons are width-dynamic by default with three settings:
*** Text beside icons that automatically collapses to icons when space is short for that particular section.
*** Text beside icons always.
*** Icons only always.
* Search and Filter toolbar has only one text entry box -- for searching within or filtering on currently displayed messages
** Toggle items in Search and Filter toolbar are sticky -- no need to pin.
** If no messages are showing, use the area to highlight which search/filtering is in place
** Search/Filtered message count is displayed in transient Status toolbar.
** Advanced Search window/dialog lives behind a menu.
* Toolbar, buttons, icons, labels, panels, columns, rows are all carefully grid aligned.
** A 9px and 14px grid for toolbars, folder pane, tree and message headers.
** Exception for Thread pane message rows being narrower.
** Exception for toggle button spacing in Search and Filter toolbar
* Color palette is simple
** Toolbar color/texture is gray
** Buttons are a tint or a shade of toolbar color/texture.
** Folder pane color/texture is light blue
** Tree and Message headers color/texture matches tree columns and message body: white.
* Server/Folder panel view is customizable only via a menu.
* Thread panel column headers are customizable via right-click on any header.
* Message panel's message header area scrolls with the message body.
* Menu toolbar is off by default.
* Status toolbar is transient like Firefox's
* Throbber is removed.
* Tabs are off by default and only enabled via pref.
(I'm not particular about the actual button and toolbar colors and styles. I've played around with a couple in this exploration. I do think flatter and more streamlined is the way to go.)