C# - Sign email - S/MIME¶

using System;
using System.Collections.Generic;
using System.Text;
using System.Security.Cryptography.X509Certificates;

// Add EASendMail namespace
using EASendMail;

namespace mysendemail
{
    class Program
    {
        static X509Certificate2 _findCertificate(string storeName, string emailAddress)
        {
            X509Certificate2 cert = null;

            X509Store store = new X509Store(storeName, StoreLocation.CurrentUser);
            store.Open(OpenFlags.ReadOnly);

            X509Certificate2Collection certfiicates = store.Certificates.Find(X509FindType.FindBySubjectName, emailAddress, true);
            if (certfiicates.Count > 0)
            {
                cert = certfiicates[0];
            }

            store.Close();

            return cert;
        }

        static void Main(string[] args)
        {
            try
            {
                SmtpMail oMail = new SmtpMail("TryIt");

                // Set sender email address, please change it to yours
                oMail.From = "test@emailarchitect.net";

                // Set recipient email address, please change it to yours
                oMail.To = "support@emailarchitect.net";

                // Set email subject
                oMail.Subject = "test email with digital signature from c#";

                // Set email body
                oMail.TextBody = "this is a test email with digital signature";

                X509Certificate2 signerCertificate = _findCertificate("My", oMail.From.Address);
                if (signerCertificate == null)
                    throw new Exception("No signer certificate found for " + oMail.From.Address + "!");

                oMail.From.Certificate2 = signerCertificate;

                // You can also load the signer certificate from a pfx file.
                /* string pfxPath = "D:\\TestCerts\\signer.pfx";
                X509Certificate2 signerCertFromPfx = new X509Certificate2(pfxPath,
                    "nosecret",
                    X509KeyStorageFlags.Exportable | X509KeyStorageFlags.UserKeySet);

                oMail.From.Certificate2 = signerCertFromPfx;
                */
                // If you use it in web application,
                // please use  X509KeyStorageFlags.Exportable | X509KeyStorageFlags.MachineKeySet

                // If you use it in .NET core application
                // please use X509KeyStorageFlags.Exportable | X509KeyStorageFlags.EphemeralKeySet

                // Your smtp server address
                SmtpServer oServer = new SmtpServer("smtp.emailarchitect.net");

                // User and password for ESMTP authentication, if your server doesn't require
                // User authentication, please remove the following codes.
                oServer.User = "test@emailarchitect.net";
                oServer.Password = "testpassword";

                // Most mordern SMTP servers require SSL/TLS connection now.
                // ConnectTryTLS means if server supports SSL/TLS, SSL/TLS will be used automatically.
                oServer.ConnectType = SmtpConnectType.ConnectTryTLS;

                // If your SMTP server uses 587 port
                // oServer.Port = 587;

                // If your SMTP server requires SSL/TLS connection on 25/587/465 port
                // oServer.Port = 25; // 25 or 587 or 465
                // oServer.ConnectType = SmtpConnectType.ConnectSSLAuto;

                Console.WriteLine("start to send email with digital signature ...");

                SmtpClient oSmtp = new SmtpClient();
                oSmtp.SendMail(oServer, oMail);

                Console.WriteLine("email was sent successfully!");
            }
            catch (Exception ep)
            {
                Console.WriteLine("failed to send email with the following error:");
                Console.WriteLine(ep.Message);
            }
        }
    }
}