It should not even touch the headers besides adding a Received-header to the top.
Re-Encoding all Mails passed through is definitely not what I call "not touching".
I know that there are some exceptions (i.e. 8bitmime), but I still think that mail servers should keep their hands of what is passing through them.
Like mailmen who are not supposed to open the envelope, read the letters and reprint them using a nicer font on nicer paper :-)