code: public void throwDirectlyEntityVariableNotAllowedException(Postcard postcard, String key, Object value) {
          final ExceptionMessageBuilder br = new ExceptionMessageBuilder();
          br.addNotice("Registered the entity directly as mail variable.");
          br.addItem("Advice");
          br.addElement("Not allowed to register register entity directly as mail variable.");
          br.addElement("Convert your entity data to mail bean for mail variable.");
          br.addElement("For example:");
          br.addElement("  (x):");
          br.addElement("    Member member = ...");
          br.addElement("    postcard.setMember(member); // *Bad");
          br.addElement("  (o):");
          br.addElement("    Member member = ...");
          br.addElement("    MemberBean bean = mappingToBean(member);");
          br.addElement("    postcard.setMember(bean); // Good");
          br.addItem("Postcard");
          br.addElement(postcard);
          br.addItem("Registered");
          br.addElement("key: " + key);
          if (value instanceof Collection<?>) {
              ((Collection<?>) value).forEach(element -> br.addElement(element));
          } else {
              br.addElement(value);
          }
          final String msg = br.buildExceptionMessage();
          throw new SMailDirectlyEntityVariableNotAllowedException(msg);
      }
docs: ===========