code: public void sendAsync() {
          if (!configuration.mailSendFilter().filter(this)) return;
          MailRequestCallbackFactory factory = configuration.mailRequestCallbackFactory();
          if (factory == null) {
              prepareSend();
              request().async().post(entity());
          } else
              sendAsync(factory.create(this));
      }
docs: Sends the email asynchronously. It uses the configuration provided
  default callback if available, ignoring the outcome otherwise.
  
  If you want to use a specific callback for this call use
  {@link #sendAsync(MailRequestCallback)} instead.