You could rename the folders, of course, to get them into more sensible an order, but why do that when you can just as well drag them into shape?
Rearrange Mail Folders in Mac OS X Mail
To change the order of mail folders in the Mac OS X Mail sidebar:
- Click the folder you want to move.
- Holding down the mouse button, drag the blue indicator where you want the folder to be.
- Release the mouse button.
(Updated July 2011)

