mail（）无法发送出去

Question

function email($reqNo){
    //Database Configurations
    $host = "example.com.au";
    $username = "example";
    $pass = "example";
    $db = "example_db";

    //Customer Configurations
    $cName = null;
    $cEmail = null;

    $DBConnect = @mysqli_connect($host, $username, $pass, $db) or die
    ("<p>Unable to connect to the database server.</p>". "<p>Error code ". mysqli_connect_errno().": ". mysqli_connect_error()). "</p>";

    $SQLString = "SELECT name, email FROM Customer WHERE cust_no = '" .$_GET["cno"]. "'";
    $queryResult = @mysqli_query($DBConnect, $SQLString) or die
    ("<p>Unable to query the $TableName table.</p>"."<p>Error code ". mysqli_errno($DBConnect). ": ".mysqli_error($DBConnect)). "</p>";

    $row = mysqli_fetch_row($queryResult);

    while ($row) {
        $cName = $row[0];
        $cEmail = $row[1];
        $row = mysqli_fetch_row($queryResult);
    }

    mysqli_close($DBConnect);

    // Email Information Configurations
    $pdate = date("Y-m-d", strtotime($_GET["p_date"]));
    $ptime = date("H:i:s", strtotime($_GET["p_time"]));
    $to = $cEmail;
    $header = "From: admin@shiponline.com";
    $subject = "Shipping Request with ShipOnline";
    $message = "Dear $cName, Thank you for using ShipOnline! Your request number is $reqNo. "
                ."The cost is " .calCost(). ". We will pick-up the item at $ptime on $pdate.";

    // Send Email
    if (mail($to, $subject, $message, $header))
        ECHO "Sent";
    else
        ECHO "Not Sent";
}

邮件功能未成功执行，因此转到else语句。我有在线资料，讲座笔记的双重检查。但仍然无法弄清楚我错过了什么或错误。

P / S：我正在使用Notepad ++进行编码。