Asyncronous mail send in vs2010 C# with delivery status

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C#

ASP.NET

hi,

I am sending bulk emails using smtp.sendasync method. Some times it shows mail status as success but the emails are not received at destination. how to get status of undelivered mails with asyncronous method. Please suggest.

What I have tried:

String eid = username;
String pass = password;
System.Net.NetworkCredential basicAuthenticationInfo = new System.Net.NetworkCredential(eid, pass);
smtpClient.Host = "smtp.rediffmailpro.com";
smtpClient.Port = 587;
smtpClient.UseDefaultCredentials = false;
// smtpClient.EnableSsl = true;
smtpClient.Credentials = basicAuthenticationInfo;
try
{
//
smtpClient.SendCompleted += (s, e) => smtpClient.Dispose();
smtpClient.SendAsync(message, null);

//smtpClient.Send(message);
return true;
}
catch (Exception exc)
{
WriteToFile("Email Send Failure: {0} " + exc.Message + ". From Email Id " + msg.Headers.From.Address.ToString());
return false;
}