Problem while sending mail

I have written the code for mail sending

public void SendMail()
    {
        try
        {
            MailMessage message = new MailMessage();
            message.From = new MailAddress(ConfigurationManager.AppSettings["FromAddress"]);
            message.To.Add(ConfigurationManager.AppSettings["ToAddress"]);
            message.Subject = "Test Mail";
            message.Body = "BX-Test Mail";
            SmtpClient client = new SmtpClient(ConfigurationManager.AppSettings["SMTPServer"], Convert.ToInt32(ConfigurationManager.AppSettings["SMTPPort"]));
            NetworkCredential credential = new NetworkCredential(ConfigurationManager.AppSettings["UserName"], ConfigurationManager.AppSettings["Password"]);         
            client.UseDefaultCredentials = false;
            client.Credentials = credential;
            client.Send(message);
        }
        catch
        {
            throw;
        }

When i deployed in server i am getting the error

Value cannot be null.
Parameter name: address 


    }