small example to parse Thunderbird messages in mbox format:

(defun thunderbird-mbox-msg-to-text (email)
  (with-open-file (msg email :direction :input)
    (with-open-file (out (concatenate 'string (pathname-name x) ".txt") :direction :output :if-exists :supersede)
      (let ((mime (parse-mime msg)))
	(case (type-of mime)
	  (text-mime (print (content mime) out))
	  (multipart-mime 
	   (dolist (part (content mime))
	     (if (and (string-equal (content-type part) "text")
		      (string-equal (content-subtype part) "plain"))
		 (print-mime out part nil nil))))
	  (mime (print "mime"))   ;?
	  (t (print "other")))))))  ;?

The MIME class is the mixin which TEXT-MIME and MULTIPART-MIME inherit from. Most plain text messages will be of type TEXT-MIME and you can read the body with the CONTENT function. MULTIPART-MIME messages have multiple parts, you will have to iterate over them inspecting the type and subtype of each and choose what to do with them. In the example above, I was extracting the plain text version of html emails. See the wikipedia page on MIME and help yourself.