Give the submit button a name:
<input type="submit" value="Submit" class="inputButton" name="submit" />
then change this line:
if ((($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST))) {
to:
if ((isset($_POST['submit']) && $_POST['submit'] != "")) {