using System.Net;
using System.Net.Mail;
using System.Net.Mime;

...
try
{

   SmtpClient mySmtpClient = new SmtpClient("my.smtp.exampleserver.net");

    // set smtp-client with basicAuthentication
    mySmtpClient.UseDefaultCredentials = false;
   System.Net.NetworkCredential basicAuthenticationInfo = new
      System.Net.NetworkCredential("username", "password");
   mySmtpClient.Credentials = basicAuthenticationInfo;

   // add from,to mailaddresses
   MailAddress from = new MailAddress("test@example.com", "TestFromName");
   MailAddress to = new MailAddress("test2@example.com", "TestToName");
   MailMessage myMail = new System.Net.Mail.MailMessage(from, to);

   // add ReplyTo
   MailAddress replyto = new MailAddress("reply@example.com");
   myMail.ReplyTo = replyto;

   // set subject and encoding
   myMail.Subject = "Test message";
   myMail.SubjectEncoding = System.Text.Encoding.UTF8;

   // set body-message and encoding
   myMail.Body = "<b>Test Mail</b><br>using <b>HTML</b>.";
   myMail.BodyEncoding = System.Text.Encoding.UTF8;
   // text or html
   myMail.IsBodyHtml = true;

   mySmtpClient.Send(myMail);
}

catch (SmtpException ex)
{
  throw new ApplicationException
    ("SmtpException has occured: " + ex.Message);
}
catch (Exception ex)
{
   throw ex;
}

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.Net.Mail;

namespace ConsoleApplication1
{
public class SendMail
{
    string[] NameArray = new string[10] { "Recipient 1",
                                          "Recipient 2",
                                          "Recipient 3",
                                          "Recipient 4",
                                          "Recipient 5",
                                          "Recipient 6",
                                          "Recipient 7",
                                          "Recipient 8",
                                          "Recipient 9",
                                          "Recipient 10"
                                        };

    public SendMail(int i, ManualResetEvent doneEvent)
    {
        Console.WriteLine("Started sending mail process for {0} - ", NameArray[i].ToString() + " at " + System.DateTime.Now.ToString());
        Console.WriteLine("");
        SmtpClient mailClient = new SmtpClient();
        mailClient.Host = Your host name;
        mailClient.UseDefaultCredentials = true;
        mailClient.Port = Your mail server port number; // try with default port no.25

        MailMessage mailMessage = new MailMessage(FromAddress,ToAddress);//replace the address value
        mailMessage.Subject = "Testing Bulk mail application";
        mailMessage.Body = NameArray[i].ToString();
        mailMessage.IsBodyHtml = true;
        mailClient.Send(mailMessage);
        Console.WriteLine("Mail Sent succesfully for {0} - ",NameArray[i].ToString() + " at " + System.DateTime.Now.ToString());
        Console.WriteLine("");

        _doneEvent = doneEvent;
    }

    public void ThreadPoolCallback(Object threadContext)
    {
        int threadIndex = (int)threadContext;
        Console.WriteLine("Thread process completed for {0} ...",threadIndex.ToString() + "at" +  System.DateTime.Now.ToString());
        _doneEvent.Set();
    }

    private ManualResetEvent _doneEvent;
}


public class Program
{
    static int TotalMailCount, Mailcount, AddCount, Counter, i, AssignI;
    static void Main(string[] args)
    {
        TotalMailCount = 10;
        Mailcount = TotalMailCount / 2;
        AddCount = Mailcount;
        InitiateThreads();

        Thread.Sleep(100000);
    }

   static void InitiateThreads()
   {
        //One event is used for sending mails for each person email id as batch
       ManualResetEvent[] doneEvents = new ManualResetEvent[Mailcount];

        // Configure and launch threads using ThreadPool:
        Console.WriteLine("Launching thread Pool tasks...");

        for (i = AssignI; i < Mailcount; i++)
        {
            doneEvents[i] = new ManualResetEvent(false);
            SendMail SRM_mail = new SendMail(i, doneEvents[i]);
            ThreadPool.QueueUserWorkItem(SRM_mail.ThreadPoolCallback, i);
        }

        Thread.Sleep(10000);

        // Wait for all threads in pool to calculation...
        //try
        //{
        // //   WaitHandle.WaitAll(doneEvents);
        //}
        //catch(Exception e)
        //{
        //    Console.WriteLine(e.ToString());
        //}

        Console.WriteLine("All mails are sent in this thread pool.");
        Counter = Counter+1;
        Console.WriteLine("Please wait while we check for the next thread pool queue");
        Thread.Sleep(5000);
        CheckBatchMailProcess();
    }

    static  void CheckBatchMailProcess()
    {

        if (Counter < 2)
        {
            Mailcount = Mailcount + AddCount;
            AssignI = Mailcount - AddCount;
            Console.WriteLine("Starting the Next thread Pool");

            Thread.Sleep(5000);
            InitiateThreads();
        }

        else
        {
            Console.WriteLine("No thread pools to start - exiting the batch mail application");
            Thread.Sleep(1000);
            Environment.Exit(0);
        }
    }
}

using System.Net.Mail

new SmtpClient("smtp.server.com", 25).send("from@email.com",
                                           "to@email.com",
                                           "subject",
                                           "body");

MailMessage message = new MailMessage(
   "fromemail@contoso.com",
   "toemail@contoso.com",
   "Subject goes here",
   "Body goes here");

SmtpClient client = new SmtpClient(server);
client.Send(message);