Here I need to keep backwards compatibility with 'destinationAddresses' using the format of previous versions using classic routing. So I am assuming if no '@' is found, we should try routing to Queue@Rebus (or whatever the fixed exchange name is).

But it would have been easier if we could simply use full destination address strings everywhere, and make it so the SubscriptionManager returned 'QueueName@Rebus' in one case, and 'WhatEver' on new-routing. But that seemed to require more hardcore changes to Rebus' core.