public static function start_phpmailer_redirection() {
        if (self::$phpmailersink) {
            // If an existing mailer sink is active, just clear it.
            self::$phpmailersink->clear();
        } else {
            self::$phpmailersink = new phpunit_phpmailer_sink();
        }
        return self::$phpmailersink;
    }