mew-unread.el adds a navigation buffer that shows a list of Mew
folders with the numbers of unread & marked messages.  When emails are
retrieved from POP server, the messages in +inbox are automatically
refiled, and Emacs jumps to *Mew unread* buffer.  Or, in summary mode,
pressing "b" will switch the current buffer to *Mew unread* buffer.

Example of "*Mew unread*" buffer

 total  unread(diff)  marked(diff)  :folder
-------------------------------------------------
  2332      25(+2)        16(+0)    :+c
  1477      55(+5)         3(+0)    :+project_clo
   236       0(+0)         0(-3)    :+private
    97       0(+0)         1(+0)    :+bill
  1237       6(+0)         0(+0)    :+ml/clsdk
   361      19(+1)         3(+0)    :+backup

The first, second and the third columns show the number of total,
unread and marked messages in each folder, respectively.  The number
in parentheses is the increased or decreased number of messages in
each folder since the last visit to the folder.  Press space or return
key to visit the folder at current cursor.

setup

*mew-unread-check-list* is a list of folders you want to see in *Mew
unread* buffer.  In your .emacs.el or .mew.el, add the following lines.

(require 'mew-unread)
(setq *mew-unread-check-list*
      '("+c"
	"+project_clo"
	"+private"
	"+bill"
	"+ml/clsdk"
	"+backup"))

If you want to see 'U' marks after refiling, set mew-use-unread-mark
non-nil, and for mew-unread-mark-list, set VALUE non-nil for selected
folders, or for all the folders as shown below.

(setq mew-use-unread-mark t)
(setq mew-unread-mark-list '((t t)))

When I wrote this, "b" was not used in Mew's key bindings.  However,
in the latest release (6.3.50), "b" was assigned to mew-summary-store.
If you want to change the key binding, modify the following part of
mew-unread.el.

(define-key mew-summary-mode-map "b" 'mew-unread-check)