Hi JC,
In addition to Jon's excelent advice above, is the contactus.php page forwarding to the thankyou.php page?
If it is not forwarding to the thankyou.php, it will indicate that the code that send s the email is not being triggered. This can happen if the host disables the HTTP_REFERER variable.
If this is the case locate the following code:
if ((($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST))) {
and change it to:
if ($_SERVER["REQUEST_METHOD"] == "POST") {