Add Reference:
  using System.Net;
  using System.Net.Mail;

Code:

      string to = "ReceiverName@gmail.com";
      string from = "SenderName@gmail.com";
      MailMessage message = new MailMessage(from, to);
      message.Subject = "Using the new SMTP client.";
      message.Body = @"Using this new feature, you can send an email message from an application very easily.";
      message.Attachments.Add(new Attachment("c:\\test.txt"));
      SmtpClient client = new SmtpClient(server, 587);
      client.EnableSsl = true;
      //client.UseDefaultCredentials = true;
      client.Credentials = new NetworkCredential("UserName", "Password");

      try
      {
        client.Send(message);
      }
      catch (Exception ex)
      {
        Console.WriteLine("Exception caught in CreateTestMessage2(): {0}", ex.ToString());
      }