Implement Incoming Iterable in the client

As I see it, the implementation of this should offer a clean way of iterating over the pending emails in the incoming box. I think here goes any abstraction that's needed to write a simple and clean iteration loop, where the logic is not mixed up with implementation details:

for item in self.incoming:
   for consumer in self.consumers:
     clear = yield consumer.decrypt(item)
     parts = yield consumer.process(clear)
     saved = yield consumer.save(parts)
   self.delete(item)

Also, any error catching belongs here.