Plain Mailboxes in Mountain Lion Mail
Plain mailboxes in OS X Mountain Lion are just like folders in the Finder; you create them and name them, and they’re empty until you put something in them. They even look like folders in Mail’s mailbox pane. You use mailboxes to organize any messages you want to save.
Here are several ways to create a plain mailbox:
Choose Mailbox→New Mailbox.
Click the little + sign at the bottom of the mailbox pane on the left side of the viewer window.
Click the Action menu at the bottom of the mailbox pane (the one that looks like a gear), and choose New Mailbox.
Right-click or Control-click in the mailbox pane and choose New Mailbox from the contextual menu.
Whichever way you choose, the next thing that happens is that a sheet drops down with a Location pop-up menu and a field for you to type the name you want to give this mailbox. Choose On My Mac from the Location menu, and name the mailbox anything you like. Click OK, and the mailbox is created in the mailbox pane.
You can create submailboxes (mailboxes inside other mailboxes) to further subdivide your messages. To do so, click a mailbox to select it before you create a new mailbox.
You can also drag and drop a mailbox from the top level of the list (such as Drafts, Junk, and Other Dumb Mail) onto another mailbox (such as Books or any of its three submailboxes) to make them submailboxes. If you drag a mailbox into a submailbox, it becomes a sub-submailbox. And so on.
To delete a mailbox, click it to select it and then
Choose Mailbox→Delete Mailbox.
Right-click or Control-click the mailbox, and choose Delete Mailbox.
Click the Action menu at the bottom of the mailbox pane (the one that looks like a gear), and choose Delete Mailbox.