@julian It's mostly append-only, easy to cache ...
For FediTest, we have come to the conclusion that simple pass/fail isn't going to cut it. We need a more gradual rating, and the current state is 1) fail -- against the spec and likely go cause interop problems 2) soft fail -- against the spec but probably harmless from an interop perspective 3) degrade -- e.g. everything turns into a Note and 4) pass. Not sure this is an exhaustive list ... your empty outbox would get a pass, I think.