!! presumes the current actor is waiting for a future receipt of a message in response to sending a message to another actor.

Sometimes though we need functionality that simply waits for any message to arrive within a timeframe. This design pattern is common in (business) process design where a 'port' is opened waiting for any of a given set of messages from a peer actor. These messages may be 'out of the blue'/not explicitly solicited. An example is in an RFQ/QUOTE negotiation scenario a seller may send RequestForClarification messages to the buyer. These messages can be sent out of the blue to the seller during the negotiation window that is time-boxed. During this timebox a receiveUntil would be used to accept RFCs, QUOTEs, etc. We continue to chain back to the receiveUntil on the seller side while the negotiation time window is open.