post = async (recipient, message) => {
      /** @param {object} allegedRecipient */
      const envelope = makeEnvelope();
      outbox.set(envelope, message);
      await E(recipient).receive(envelope, selfId);
      // Send to own inbox.
      if (message.from !== message.to) {
        deliver(message);
      }
    }
@param {Handle} recipient
@param {EnvelopedMessage} message