Rewriting the Content-Type header with exim/procmail?

["Marc Sherman" <msherman@projectile.dyndns.org>]

When I read pgp-signed mail sent from Mutt with Outlook
Express 5 on a windows box, outlook interprets the body
of the message as a text attachment, and displays no
body for the message.

I suspect that this is because Mutt is sending the message
with the Content-Type header as:
Content-Type: multipart/signed; micalg=pgp-md5;
 protocol="application/pgp-signature"; boundary="some_random_junk"

while Outlook sends (and presumably expects to receive)
mail with the Content-Type header as:
Content-Type: multipart/mixed;
 boundary="----=_NextPart_000_005C_01BF77BC.205C4C60"

My mail is being sent to a debian (frozen) machine
running exim, and I'm reading it on a windows box
using pop3.  Is there any filter I can install in 
exim (or procmail) on the debian machine which can
take mail with a Content-Type of mulitpart/signed,
and rewrite that header as Content-Type: multipart/
mixed, dropping the micalg and protocol attributes
but keeping the same boundary attribute?

Thanks for any pointers,

- Marc