Thanks, this is helpful.
To be clear in the email example protonmail, gmail are “apps” in the eyes of the email customers.
Relay would be something like set up your POP3, or SMTP servers. These technicalities are what gmail handles on your behalf, so gmail is running the outbox equivalent for its customers.
I love the idea upon outbox things can just be found and just work (has any dev measured this statement yet?). Agree directionally this is correct.
Individual censorship decisions for an entire relay is a good point for having to think about a relay - maybe this a future solution to a future problem? Is it a today problem?