code: public void sendAll(String apiKey, Map<String, ? extends Collection<?>> messagesByQueue) {
          checkNotNull(messagesByQueue, "messagesByQueue");
          if (messagesByQueue.isEmpty()) {
              return;
          }
          if (messagesByQueue.size() == 1) {
              // Prefer the single queue REST call because it'll log the queue name in the HTTP server request logs.
              Map.Entry<String, ? extends Collection<?>> entry = Iterables.getOnlyElement(messagesByQueue.entrySet());
              sendAll(apiKey, entry.getKey(), entry.getValue());
              return;
          }
          try {
              URI uri = _queueService.clone()
                      .segment("_sendbatch")
                      .build();
              _client.resource(uri)
                      .type(MediaType.APPLICATION_JSON_TYPE)
                      .header(ApiKeyRequest.AUTHENTICATION_HEADER, apiKey)
                      .post(messagesByQueue);
          } catch (EmoClientException e) {
              throw convertException(e);
          }
      }
docs: Any server can handle sending messages, no need for @PartitionKey