Si quieres añadir a los comentaristas de tu web a las listas de MailPoet (con el permiso correspondiente), usa este código:

add_action('wp_insert_comment', 'suscribir_autores_comentarios_mailpoet', 10, 2);
function suscribir_autores_comentarios_mailpoet($comment_id, $comment_object) {
    // Obtener la información del comentario
    $comment_author_email = $comment_object->comment_author_email;
    $comment_author_name = $comment_object->comment_author;

    if ( class_exists(MailPoetAPIAPI::class ) ) {
        $mailpoet_api = MailPoetAPIAPI::MP('v1');

        // Reemplaza los 'ID_DE_TUS_LISTAS' con los IDs de las listas de Mailpoet a las que deseas añadir los usuarios.
        $lists = array(7,9,6,8,4,5,11); //La 11 es "Comentarios WP"
        $user_data = get_userdata( $user_id );
        $sub = array(
            'email' => sanitize_email($comment_author_email),
            'first_name' => sanitize_text_field( $comment_author_name ),
        );

        // See if the user exists first.
        try {
            $subscriber = $mailpoet_api->getSubscriber( $sub['email'] );
        } catch ( Throwable $th ) {
            // If the user doesn't exist, create them.
            $subscriber = $mailpoet_api->addSubscriber( $sub    );
        }


        // Try add the user to lists.
        if ( $subscriber ) {

            $user_id = $subscriber['id'];
        
            // add users to the lists.
            try {
                $subscribe = $mailpoet_api->subscribeToLists( $user_id, $lists );
            } catch ( Throwable $th ) {
                echo 'unable to add to lists - ' . $th;
            }
        }
    }

}