Setting up in- and outbox relays is unlikely to ever be good UX. 

The more relays start to behave like communitues, the more silly all this will look, and the easier it all is for normies to grasp.

It makes sense for people to have an inbox-solution, using 1 or more relays. 

But who in his right mind targets all of their publications to the exact same relays (i.e. Communities)?