Rewriting is a common cause of breaking DKIM signatures.  There is a 
plethora of cases where they fail anyway, but in this case the message 
was a plain text message in us-ascii, so it's quite difficult to 
understand why signatures have been broken.

My understanding is that the message was rewritten because it missed 
an explicit "Content-Transfer-Encoding: 7bit".  I see two ways of 
fixing this with one liners:

* In submit2.c, MessageEnd, ~981: don't rewrite the message if there
   are no serious reasons to do it.  This is the most DKIM-friendly
   change.

                if (rfc2045_ac_check(rwrfcptr, rwmode))
                        dorewrite=mime? 1: 0;

* In rfc2045acchk.c, rfc2045_ac_check, ~71: if the message misses a
   C-T-E field but doesn't actually need one, don't rewrite it.  This
   is the most case-specific change.

        if (RFC2045_ISMIME1DEF(p->mime_version)
                && !p->content_transfer_encoding
                && hasnon7bit)

   and later, ~102, check again...

                te=p->content_transfer_encoding;
                is8bitte=te && strcasecmp(te, "base64") &&

(All snippets terminate with the changed line.)



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