OS X Mail, like other Macintosh OS X programs, opens within a window and includes
the standard control buttons. A menu bar appears at the top of the window. The
standard toolbar appears under the menu bar.
Menu Bar
Toolbar
The sidebar appears on the side (If it is not there, click Mailboxes on the Toolbar).
It contains shortcuts to mailboxes/folders. You can add and remove shortcuts, make the shortcuts
small or large, delete or add mailboxes.
Mailboxes Sidebar
The main window of OS X Mail displays items of the mailbox/folder/shortcut you have selected.
The gray bar that spans the top of the main window displays the name of the current selection as shown in
the example of the In box below.