我正在编写一个程序，从一封邮件发送数据库中的多封用户邮件，但我得到一个错误
对象引用未设置为对象的示例。
下面是连接到db的代码。

SqlCommand cmd = null;

            // Error occurs on the line below
            string connectionString = (ConfigurationManager.ConnectionStrings["SchedulingInterviewsConnectionString"].ConnectionString); 
            string queryString = @"SELECT Email FROM interview WHERE Email = Email";

            using (SqlConnection connection =
                       new SqlConnection(connectionString))
            {
                SqlCommand command =
                    new SqlCommand(queryString, connection);
                connection.Open();
                cmd = new SqlCommand(queryString);
                cmd.Connection = connection;

                SqlDataReader reader = cmd.ExecuteReader