How Can i Send Mail sending Through Exchange Server by using SMTP

I want to Run Below code without NetworkCredential nc = new Net.NetworkCredential("USERNAME", "PASSWORD"). 

BY using Only Exchange Host (Server Name) And Port 

Im Getting Error For this code : Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender

 protected void SendEmail(object sender, EventArgs e)
    {
        SmtpClient smtpClient = new SmtpClient("ExchangeServerName",25);
        MailMessage message = new MailMessage();
        try
        {
            MailAddress fromAddress = new MailAddress("bala@OfficeName.com", "From Me");
            MailAddress toAddress = new MailAddress("bala@OfficeName.com", "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("ExchangeServerNameSoll", 25, "Basic");
            smtpClient.Send(message);
            lblText.Text ="Email sent.";
        }
        catch (Exception ex)
        {
            lblText.Text = "Coudn't send the message!\n  " + ex.Message;
        }
    }