code: public void sendReceipt(SignalServiceAddress recipient,
                            Optional<UnidentifiedAccessPair> unidentifiedAccess,
                            SignalServiceReceiptMessage message)
        throws IOException, UntrustedIdentityException
    {
      byte[] content = createReceiptContent(message);
  
      sendMessage(recipient, getTargetUnidentifiedAccess(unidentifiedAccess), message.getWhen(), content, false);
    }
docs: Send a read receipt for a received message.
  
  @param recipient The sender of the received message you're acknowledging.
  @param message The read receipt to deliver.
  @throws IOException
  @throws UntrustedIdentityException