code: public static void useSurrogateHolder(SMailCallbackContextHolder holder) {
          assertNotLocked();
          if (_log.isInfoEnabled()) {
              _log.info("...Setting surrogate holder for callback context: " + holder);
          }
          if (holder != null) {
              _holder = holder;
          } else {
              _holder = _defaultHolder;
          }
          _locked = true;
      }
docs: Use the surrogate holder for callback context. (automatically locked after setting) <br>
  You should call this in application initialization if it needs.
  @param holder The holder instance. (NullAllowed: if null, use default holder)