@Override
  protected void route(List<SendingMailbox> sendingMailboxes, TransferableBlock block)
      throws Exception {
    sendBlock(sendingMailboxes.get(0), block);
  }
@Test(expectedExceptions = IllegalArgumentException.class)
  public void shouldThrowWhenSingletonWithMultipleMailboxes()
      throws Exception {
    // Given:
    ImmutableList<SendingMailbox> destinations = ImmutableList.of(_mailbox1, _mailbox3);

    // When:
    new SingletonExchange(destinations, TransferableBlockUtils::splitBlock).route(destinations, _block);
  }