The $name, $email and $message variables are outside of the scope of the function sendmail. You either have to put the following line at the top of your function:

global $name, $email, $message;

or add arguments to the function:

function sendmail($name, $email, $message) {
    ...
}

which would change how to you call it:

sendmail($_POST['name'], $_POST['email'], $_POST['message']);

Hope that helps!