使Outlook识别我的数字签名电子邮件

我正在使用c# .NET 4.0发送签名SMTP邮件消息，如下所示:

    private void SendMailMessage(object data)
    {
        MailMessage message = new MailMessage();
        message.From = new MailAddress(fromAddress);
        message.To.Add(new MailAddress(emailTo));
        message.Subject = "Subject";
        message.IsBodyHtml = true;
        message.Body += "Blah blah blah.";
        byte[] messageBytes = Encoding.ASCII.GetBytes(message.Body);
        SignedCms Cms = new SignedCms(new ContentInfo(messageBytes));
        CmsSigner Signer = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, certificate);
        Cms.ComputeSignature(Signer);
        byte[] SignedBytes = Cms.Encode();
        MemoryStream signedStream = new MemoryStream(SignedBytes);
        AlternateView signedView = new AlternateView(signedStream, "application/pkcs7-mime; smime-type=signed-data;name=sig.p7m");
        message.AlternateViews.Add(signedView);
        SmtpClient client = new SmtpClient(smtpServer, int.Parse(smtpServerPort));
        client.DeliveryMethod = SmtpDeliveryMethod.Network;
        client.Send(message);
        message.Dispose();
        client = null;       
    }

据我所知，这是"有效的"，因为如果查看消息的原始数据，我看到其中有一个大PKCS签名的备用视图。但Outlook无法识别它。Outlook客户端通常会识别已签名的消息，并尝试验证它们，并在消息上添加一个小证书等等…

我想…我错过了什么?

编辑:我自己在这方面取得了一些进展，但仍然有一些麻烦。下面是代码现在的样子:

    private void SendMailMessage(string emailTo)
    {            
        MailMessage message = new MailMessage();
        message.From = new MailAddress(fromAddress);
        message.To.Add(new MailAddress(emailTo));
        message.Subject = "Special Delivery";
        message.IsBodyHtml = false;
        string body = "Content-Type: text/plain;charset='"iso-8859-1'"'nContent-Transfer-Encoding: quoted-printable'n'nHere is some body text!";
        byte[] messageBytes = Encoding.ASCII.GetBytes(body);
        ContentInfo content = new ContentInfo(messageBytes);
        SignedCms signedCms = new SignedCms(content, false);
        CmsSigner Signer = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, certificate);
        signedCms.ComputeSignature(Signer);
        byte[] signedBytes = signedCms.Encode();
        MemoryStream ms = new MemoryStream(signedBytes);
        AlternateView av = new AlternateView(ms, "application/pkcs7-mime; smime-type=signed-data;name=smime.p7m");
        message.AlternateViews.Add(av);
        SmtpClient client = new SmtpClient(smtpServer, int.Parse(smtpServerPort));
        client.DeliveryMethod = SmtpDeliveryMethod.Network;
        client.Send(message);
        message.Dispose();
        client = null;
    }

值得注意的是我留了言。这次主体空白，只发送AlternateView。现在，当我把它发送到Outlook收件箱时，我的电子邮件上就会出现挂锁图标，S/MIME工具会启动并试图验证签名者，但失败了。用于签名电子邮件的证书是由公共信任的CA颁发的。编辑:这是我的错，证书没有"安全电子邮件"使用属性。我去办个新证书。

当我将相同的电子邮件发送到Gmail地址时，我收到一个带有*的空白消息。P7m附件里面有一堆垃圾

private void SendMailMessage(string emailTo)
{
    MailMessage message = new MailMessage();
    message.From = new MailAddress(fromAddress);
    message.To.Add(new MailAddress(emailTo));
    message.Subject = "Regarding your lottery winnings";
    message.IsBodyHtml = false;
    string body = "Content-Type: text/plain;charset='"iso-8859-1'"'nContent-Transfer-Encoding: quoted-printable'n'nBlah blah blah blah blah blah.";                
    byte[] messageBytes = Encoding.ASCII.GetBytes(body);
    ContentInfo content = new ContentInfo(messageBytes);
    SignedCms signedCms = new SignedCms(content, false);
    CmsSigner Signer = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, emailCert);
    signedCms.ComputeSignature(Signer);
    byte[] signedBytes = signedCms.Encode();
    MemoryStream ms = new MemoryStream(signedBytes);
    AlternateView av = new AlternateView(ms, "application/pkcs7-mime; smime-type=signed-data;name=smime.p7m");
    message.AlternateViews.Add(av);                
    SmtpClient client = new SmtpClient(smtpServer, int.Parse(smtpServerPort));
    client.DeliveryMethod = SmtpDeliveryMethod.Network;
    client.Send(message);
    message.Dispose();
    client = null;
}