(setq jk-header-intro-mapping 
      '(("AW\\|SV\\|RE" "Re") 
	("WG"      "Fwd")))

(defun jk-match-header-intro (mapping intro)
  (if mapping
      (if (string-match (car (car mapping)) intro)
	  (car (cdr (car mapping)))
	(jk-match-header-intro (cdr mapping) intro))
    nil))

(setq jk-header-change-all t)

(defun jk-change-outlook-headers ()
  "Change Subject according to jk-header-intro-mapping
     in mails produced by Microsoft Outlook."
  (save-excursion
    (goto-char (point-min))
    (if (or jk-header-change-all
	    (re-search-forward 
	     "^X-Mailer: \\(Internet Mail Service\\|Microsoft\\|Outlook\\)" nil t))
	(progn
	  (goto-char (point-min))
	  (if (re-search-forward 
	       "^\\(Subject: \\(=\\?[^?]+\\?[^?]+\\?\\)?\\)\\(\\w\\{1,3\\}\\)\\(:\\|=3A\\)" nil t)
	      (let ((regexp-state (match-data)))
		(let ((replacement (jk-match-header-intro jk-header-intro-mapping
							  (match-string 3))))
		  (if replacement
		      (progn
			(set-match-data regexp-state)
			(replace-match (concat 
					(match-string 1) 
					replacement 
					(match-string 4)) t))))))))))

(add-hook 'nnmail-prepare-incoming-header-hook 'jk-change-outlook-headers)