code: public void setSubject(String subject, String encoding) {
          assertArgumentNotNull("encoding", encoding);
          saveSubject(subject);
          try {
              message.setSubject(subject, encoding);
          } catch (MessagingException e) {
              String msg = "Failed to set subject: " + subject + " message=" + message;
              throw new SMailMessageSettingFailureException(msg, e);
          }
      }
docs: ------