wordpress wp_mail() function with jquery $.post()

[admin]

I'm building custom contact form on wordpress using jquery $.post() function.

$.post(tmpl_dir + '/bugreport.php',{ name:name, email:email, message:message }, function(data) {
                if ( data ) alert( 'thanks for answer' )
                else alert('error sending, please try again.');

            })

(tmlp_dir is theme directory path)

here is bugreport.php file:

<?php 
if ( isset($_POST['name']) && isset($_POST['email']) &&isset($_POST['message']) ) {
    $name = $_POST['name']; $email = $_POST['email']; $message = $_POST['message'];
    $send_to = "[email protected]";
    $subject = "Question from " . $name;
    $success = wp_mail($send_to,$subject,$message);
            if ($succsess) return true
            else return false;
}
?>

I'm getting an error from wordpress that says: wp_mail() function not defined.
How can i let my php file use wp_mail() function?

Thanks.