Quiero ejecutar el código de abajo sin

NetworkCredential nc = new Net.NetworkCredential("USERNAME", "PASSWORD"). 

Utilizando solo Host de Exchange (nombre del servidor) y el puerto

Estoy obteniendo un error para este código: buzón no disponible. La respuesta del servidor fue: 5.7.1 El cliente no tiene permisos para enviar como este remitente

protected void SendEmail(object sender, EventArgs e)
{
    SmtpClient smtpClient = new SmtpClient("ExchangeServerName",25);
    MailMessage message = new MailMessage();
    try
    {
        MailAddress fromAddress = new MailAddress("[email protected]", "From Me");
        MailAddress toAddress = new MailAddress("[email protected]", "To You");
        message.From = fromAddress;
        message.To.Add(toAddress);
        message.Subject = "Testing!";
        message.Body = "This is the body of a sample message";
        smtpClient.UseDefaultCredentials = true;
        System.Net.NetworkCredential nc = CredentialCache.DefaultNetworkCredentials;
        smtpClient.Credentials = (System.Net.ICredentialsByHost)nc.GetCredential("ExchangeServerName", 25, "Basic");
        smtpClient.Send(message);
        lblText.Text ="Email sent.";
    }
    catch (Exception ex)
    {
        lblText.Text = "Coudn't send the message!\n  " + ex.Message;
    }
}