Wordpress function notify-mail admin

I try to modify a function in a wordpress plugin, who notify administrators by email, that a new post on bbPress forum or answer are created.
I managed to have multiple recipients.
Now I want to have the email of the user who post in the forum.
Actually, I have the display name of the poster.

I try this :

function notify_admin($post_id) {

    if (get_option(self::TD . 'notify')) {
        $multiple_recipients = array(
        'xxx@xxxx.xxx',
        'xxx@xxxx.xxx',
        'xxx@xxxx.xxx'
        );
        $blogname = wp_specialchars_decode(get_option('blogname'), ENT_QUOTES);
        $blogurl = get_option('home');
        $message  = sprintf(__('Nouveau sujet ou réponse en attente de moderation sur le site %s: %s', self::TD), $blogname, $blogurl) . "\r\n\r\n";

        /* Add body of topic/reply to email */
        $post = get_post($post_id);
        $title = '';

        if ($post) {
            $title = $post->post_title;
            $message .= get_permalink($post->ID) . "\r\n\r\n";
            $author = get_userdata($post->post_author); //HERE I WANT TO GET THE USER EMAIL
            $email = get_userdata($post->user_email);
            $message .= "Le contenu suivant a été posté\r\n";
            if ($author !== false) {
                $name = $author->user_firstname . " " . $author->user_lastname;
                $name = trim($name);
                if (empty($name)) {
                    $name = $author->display_name;
                }
                if ($name == $author->user_login) {
                    $name = '';
                } else {
                    $name = ' (' . $name . ')';
                }
                $message .= "Par " . $author->user_login .  $name . "\r\n\r\n";
            } else {
                $message .= "Par Anonyme\r\n\r\n";
            }
            $message .= $post->post_title . "\r\n" . $post->post_content . "\r\n\r\n";
            $message .= "Pour modérer, cliquez ici : xxxxxxx";
            $message .= $email . "\r\n"; //HERE I WANT TO PUT IT ON MY MAIL
        }

        @wp_mail(($multiple_recipients), sprintf(__('[%s] Moderation Forum', self::TD), $blogname, $title), $message);
    }
}

With that, I don't have the $email fiel in my mail.
I'm not an expert on PHP, sorry if I do mistakes.
Can you help me please ?