namespace BlockSending
{
    class Program
    {
        static void Main(string[] args)
        {
            MailSending send = new MailSending();
            send.Test();
        }
    }

    class MailSending
    {
        BlockingCollection<string> PendngMails = new BlockingCollection<string>() ;
        public void Test()
        {
            for (int i = 0; i < 4; i++)
            {
                PendngMails.Add("ivan@mydomain.com");
            }
            PendngMails.CompleteAdding();

            Task[]  tasks = new Task[10];
            for (int i = 0; i < 10; i++)
            {
                tasks[i] = Task.Factory.StartNew(() =>
                {

                    while (!PendngMails.IsCompleted)
                    {

                        SmtpClient mailSmtp = null;
                        try
                        {
                            var item = PendngMails.Take();

                            mailSmtp = new SmtpClient();


                            mailSmtp.RcptToErrorContinue = true;
                            mailSmtp.Tag = item;

                            SmtpMail oMail = new SmtpMail("TryIt");

                            string defultsender = "Dummy Email";
                            oMail.From = new MailAddress("" + defultsender + "<" + item + ">");

                            oMail.To = item;
                            oMail.Subject = "Test Email";
                            oMail.TextBody = "This is test email.";
                            SmtpServer oServer = new SmtpServer("localhost");
                            oServer.Protocol = ServerProtocol.SMTP;



                            oServer.ConnectType = SmtpConnectType.ConnectSSLAuto;

                            //mailSmtp.LogFileName = @"C:\Log.txt";
                            mailSmtp.SendMail(oServer, oMail);

                            oMail.To.Clear();
                            Console.WriteLine("Mail sent");

                           // _CrossThreadSetItemText(mailSmtp.Tag, "Success");
                        }
                        catch (Exception ex)
                        {

                           // _CrossThreadSetItemText(mailSmtp.Tag, ex.Message);
                        }
                    }

                });


               
            }
            System.Threading.Tasks.Task.WaitAll(tasks);
        }
    }
}