Ever heard of BCC (Blind Carbon Copy) ? :)
If you can make sure that your SMTP Client can add the addresses as BCC, then your problem will be solved :)
There seems to be a Blind Carbon Copy item in the MailMessage class
http://msdn.microsoft.com/en-us/library/system.net.mail.mailmessage.aspx
http://msdn.microsoft.com/en-us/library/system.net.mail.mailmessage.bcc.aspx
Here is a sample i got from MSDN
public static void CreateBccTestMessage(string server)
{
MailAddress from = new MailAddress("[email protected]", "Ben Miller");
MailAddress to = new MailAddress("[email protected]", "Jane Clayton");
MailMessage message = new MailMessage(from, to);
message.Subject = "Using the SmtpClient class.";
message.Body = @"Using this feature, you can send an e-mail message from an application very easily.";
MailAddress bcc = new MailAddress("[email protected]");
//This is what you need
message.Bcc.Add(bcc);
SmtpClient client = new SmtpClient(server);
client.Credentials = CredentialCache.DefaultNetworkCredentials;
Console.WriteLine("Sending an e-mail message to {0} and {1}.",
to.DisplayName, message.Bcc.ToString());
try {
client.Send(message);
}
catch (Exception ex) {
Console.WriteLine("Exception caught in CreateBccTestMessage(): {0}",
ex.ToString() );
}
}