code: public Observable<Mail> starMail(int mailId, final boolean star) {
  
      return getMail(mailId).map(new Func1<Mail, Mail>() {
        @Override public Mail call(Mail mail) {
          mail.setStarred(star);
          return mail;
        }
      });
    }
docs: Star or unstar a mail
  
  @param mailId the id of the mail
  @param star true, if you want to star, false if you want to unstar