code: @Override
      public Response sendEmail(String CorpNum, MgtKeyType KeyType,
                                String MgtKey, String Receiver) throws PopbillException {
          return sendEmail(CorpNum, KeyType, MgtKey, Receiver, null);
      }
docs: /* (non-Javadoc)
  @see com.popbill.api.TaxinvoiceService#sendEmail(java.lang.String, com.popbill.api.taxinvoice.MgtKeyType, java.lang.String, java.lang.String)