Smart Mailboxes in Mountain Lion Mail
A smart mailbox is Mail’s version of the Finder’s Smart Folder. In a nutshell, smart mailboxes OS X Mountain Lion's Mail application are mailboxes that display the results of a search. The messages you see in a smart mailbox are virtual; they aren’t really in the smart mailbox itself.
Instead, the smart mailbox displays a list of messages stored in other mailboxes that match whatever criteria you’ve defined for that smart folder. As with Smart Folders in the Finder, smart mailboxes update automatically when new messages that meet the criteria are received.
Here are two ways to create a smart mailbox:
Choose Mailbox→New Smart Mailbox.
Click the + button at the bottom of the mailbox pane, and choose Smart Mailbox from the pop-up menu.
Whichever way you choose, the next thing that happens is that a sheet drops down with a field for the smart mailbox’s name, plus some pop-up menus, buttons, and check boxes.
Name your smart mailbox; determine its criteria (by using the pop-up menus, plus and minus buttons, and check boxes), and then click OK. The smart folder appears in the mailbox pane with a little gear on it to denote that it’s smart.
You can see the Smart Mountain Lion smart mailbox in the figure, though only the first part of its name — Smart Mountain — is visible. Notice that it has a gear, and plain mailboxes don’t.