Parallel Email sending with throttling.

gistfile1.cs

pyblic async Task SendEmailsWithThrottling(List<string> totalRecipientList, int throttle) 
{
  for(var i = 0; i < Math.Ceil(totalRecipientList.Count / throttle); i++)
  {
    var recipientList = totalRecipientList.Skip(i * throttle).Take(throttle);
    var sendEmailTasks = recipientList.Select(async recipient =>
    {
        var message = new MailMessage { To = { recipient }, ... };
     
        // instantiate a new client for each mail because of this:
        // http://www.codefrenzy.net/2012/01/30/how-asynchronous-is-smtpclient-sendasync/
        using (var smtpClient = new SmtpClient())
        {
            await smtpClient.SendMailAsync(message);
        }
    });
    await Task.WhenAll(sendEmailTasks);
  }
}