I dont think its just a header, there is a header:
Content-Type: multipart/signed;
protocol="application/x-pkcs7-signature";
micalg=SHA1;
boundary="----=_NextPart_XXX_XXXX_XXXXXXXX.XXXXXXXX"
And one file attached smime.7p (or something).
I will try this weekend to do this manually.
The software dont have this feature, right?
Thanks.