code: protected void hookPreparedMessage(Postcard postcard, final SMailPostingMessage message) {
          if (SMailCallbackContext.isExistPreparedMessageHookOnThread()) {
              final SMailCallbackContext context = SMailCallbackContext.getCallbackContextOnThread();
              final SMailPreparedMessageHook hook = context.getPreparedMessageHook();
              hook.hookPreparedMessage(postcard, message);
          }
      }
docs: ================