you'll actually need to host it yourself. It's going to be a one-to-one relationship between mailboxes and briar users on purpose to avoid centralization of any kind (think techies hosting mailboxes for many friends etc.). To make hosting rather easy, the mailbox can be run on a spare (old) Android device, ideally plugged into a power outlet and connected to Wifi. Setup will include installing the app onto the Android and going through a pairing procedure by scanning a QR-code from the mailbox app from the briar settings screen.