I have a much smaller set of boxes - 12 boxes for me and my neighbors (all single-family homes). We don’t have an outgoing box, and our “signal” for outgoing mail is to place it in the box vertically/upright rather than horizontally/laying flat. Since the postman puts everything flat, he knows that anything placed vertically was put there by us.
On the other hand, he does miss it a couple times a year and I’ve taken to dropping mail off at the office where we have a dedicated outgoing slot.