[mew-int 3118] 'U' mark can only use in inbox

[chunhui]

hi every one
   I set the 'U' mark in the .mew.el like this: 
         (setq mew-use-unread-mark t)      
but the 'U' can only display in the inbox. other imap folders are not display.
when there are new mails.
 
  why? and how can I set the .mew.el to display the 'U' in all folders?
  thanks.