public static void RetryIfBusy(string server)
{
    MailAddress from = new MailAddress("[email protected]");
    MailAddress to = new MailAddress("[email protected]");
    MailMessage message = new MailMessage(from, to);
    // message.Subject = "Using the SmtpClient class.";
    message.Subject = "Using the SmtpClient class.";
    message.Body = @"Using this feature, you can send an email message from an application very easily.";
    // Add a carbon copy recipient.
    MailAddress copy = new MailAddress("[email protected]");
    message.CC.Add(copy);
    SmtpClient client = new SmtpClient(server);
    // Include credentials if the server requires them.
    client.Credentials = (ICredentialsByHost)CredentialCache.DefaultNetworkCredentials;
    Console.WriteLine("Sending an email message to {0} using the SMTP host {1}.",
         to.Address, client.Host);
    try
    {
        client.Send(message);
    }
    catch (SmtpFailedRecipientsException ex)
    {
        for (int i = 0; i < ex.InnerExceptions.Length; i++)
        {
            SmtpStatusCode status = ex.InnerExceptions[i].StatusCode;
            if (status == SmtpStatusCode.MailboxBusy ||
                status == SmtpStatusCode.MailboxUnavailable)
            {
                Console.WriteLine("Delivery failed - retrying in 5 seconds.");
                System.Threading.Thread.Sleep(5000);
                client.Send(message);
            }
            else
            {
                Console.WriteLine("Failed to deliver message to {0}", 
                    ex.InnerExceptions[i].FailedRecipient);
            }
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine("Exception caught in RetryIfBusy(): {0}", 
                ex.ToString() );
    }
}