Send multipart/signed email c#

Question

I am trying to create a multipart/signed mime email following the RFC1847 protocol. This is how it is supposed to look (part of the signature is removed):

Content-Type: multipart/signed; protocol="application/pkcs7-signature"
    micalg=sha1; boundary="--PTBoundry=3"

----PTBoundry=3
Content-Type: multipart/mixed;
boundary="--PTBoundry=2"

----PTBoundry=2
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

TEST AF signed
----PTBoundry=2
Content-Type: application/octet-stream;
    name=test2.txt
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
    filename=test2.txt

bWludGVzdGF0dGFjaG1lbnRzaWduZWQ=
----PTBoundry=2--

----PTBoundry=3
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"

MIIKfAYJKoZIhvcNAQcCoIIKbTCCCmkCAQExCzAJBgUrDgMCGgUAMAsGCSqGSIb3DQEHAaCCCNow
ggPVMIICvaADAgECAgMCNtEwDQYJKoZIhvcNAQEFBQAwQjELMAkGA1UEBhMCVVMxFjAUBgNVBAoT
PrENekpgrYkz

----PTBoundry=3--
empty line

I cannot figure out how to actually send this as an email. I am using MailMessage and I have tried to add it as written below:

var stream = new MemoryStream(Encoding.ASCII.GetBytes(message));
var view = new AlternateView(stream, "application/pkcs7-mime; smime-type=signed-data;name=smime.p7m");

however it does not work. The MailMessage adds different headers and messes it all up.

How can I send this correctly?

Answer 1

I am no longer using the MailMessage class, because I could not get it working. Instead I am using EWS managed api like this:

var mail = new EmailMessage(_service);
mail.Subject = filename;
mail.MimeContent = new MimeContent("us-ascii", Encoding.ASCII.GetBytes(messageContentWithHeaders));
mail.SendAndSaveCopy();