To read flow-formatted messages in Wanderlust put the following in your .wl file (you need flow-fill.el which included in the current emacs release):

 ;; Reading f=f
 (autoload 'fill-flowed "flow-fill")
 (add-hook 'mime-display-text/plain-hook
 	  (lambda ()
 	    (when (string= "flowed"
 			   (cdr (assoc "format"
 					(mime-entity-content-type entity)))))

If you want to manually write format=flowed messages, just add “--[[text/plain; format=flowed]] at the beginning of your message:

 (defun my-mail-setup ()
     (mime-edit-insert-tag "text" "plain" "; format=flowed")))
 (add-hook 'wl-mail-setup-hook 'my-mail-setup)

In order to automatically format messages as format=flowed, download format-flowed.el and say:

  (add-hook 'mime-edit-translate-hook 'format-flowed-translate-message)

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