Courier

Delivering messages by Courier is the most sure way to receive a response. If you trust a message to a Courier, they can ensure they place the message into the hands of the intended recipient, and inform them if a response is expected.

However, most couriers have been bought by one House or another, and you can never be sure that a message sent by courier hasn't been read by at least one other person than the intended recipient's household.

If you'd like a little more security, but a bit slower response time, you might look into using the Pneumatic.