Leave a note
You tap, your mail app opens with the note already addressed. You write it, you send it, the neighbor reads it on their own phone — and no computer in between ever holds a copy it should not have.
Every door is a post box
The note board
Pinned items live in the building's book — the pages people actually turn. If a note outlasts a day, it belongs on the board: "what we share" and "where our group lives" are post boxes already.
The doorbell
If it can't wait, say hi face to face. Two phones, direct, encrypted, no relay — and if the hall is strict Wi-Fi, the doorbell simply doesn't ring instead of sneaking through anyone else's wires.
The seal
Notes that matter to the building's words get a written copy at the agreement desk. A promise on paper beats a promise in the group chat — copies must agree.
Why the postbox is your mail app, not a relay
Presence needs a page somewhere — that is the one place a middleman is wanted and named. But it does not need a new one. A relay would mean running (and watching) a store for hundreds of buildings; your own mail app already runs one per household, and the only reader of a note between two neighbors is the post office they already have.
The line stays drawn: the hello is direct, the note travels by your own mail, and the book holds what the building agrees on. No TURN relay, no company postbox, ever, for now.