How to know whether to trust a red bPost mailbox

bPost mailboxes throughout Brussels are littered with stickers and graffiti. They look abandoned. Some boxes have a placard that shows the last collection time of the day, and some do not. If they don’t have the placard, does that mean the box is out of service?

I also wonder how the abandoned ones could be of use. Maybe if you have a legal obligation to send something but you don’t want to, you could use one of these possibly abandoned boxes just to have a courtroom defense.