Is it possible to send an email to a partner with the XmlRpc API? I tried using the following PHP code, but I can't figure out how to compose the arguments.

$id = $client->call('res.partner', 'message_post', array(
        $parterId,
        'body',
        'subject'
    ));

This call succesfully posts a message to the user's document but sends no email.

Am I using the right model? What arguments should I pass?