2 years ago leora has asked this question and there is no answer for Outlook 2003 & 2007.
Does anyone know how to do it in Outlook 2010 or just "Could a newer and better mail client do this simple thing"?
In fact, there is a tip to do this thing, but quite dummy and personally, I hate to do these things manually for any mail box I have.
Search Folders -
New Search Folder... -
Unread Mail -
Right-click on new
Unread Mail (may be Unread1 if there is another Unread box) --
Customize this Search Folder -
Browse... -- choose the folder you want to show unread count and don't forget the check box Search sub folder.
Right-click and choose Show in Favorites to get focus.