As a service bus, I want to use a generic envelope factory interface to build envelopes for communicating with endpoints so that I can obtain a level of isolation between the Envelope class and the implementation.

1. When creating an envelope using the factory, I expect to create an IBuildEnvelope interface in response.

2. When receiving an envelope from an endpoint, I expect to receive an IEnvelope interface.

3. When creating an Endpoint, I expect to acquire an IEnvelopeBuilder interface from the Endpoint that I will use to create envelopes destined for that Endpoint.

Actual implementation:

IEnvelopeFactory, supported by MessageQueueEndpoint, used for creation of envelopes by the service bus.