code: @Override
      public SentMessage[] getMessages(String CorpNum, String receiptNum)
              throws PopbillException {
          if (receiptNum == null)
              throw new PopbillException(-99999999, "접수번호가 입력되지 않았습니다.");
  
          return httpget("/Message/" + receiptNum, CorpNum, null,
                  SentMessage[].class);
      }
docs: /*
  (non-Javadoc)
  @see com.popbill.api.MessageService#getMessages(java.lang.String, java.lang.String)