You should pay attention to this when placing your letterbox:

You must use the mailbox at the edge of the public place away. The apartment blocks with more than four buses are an exception. Here, the mailboxes must be on the ground floor and be easily accessible. The house numbers or bus numbers must always be visible and readable from public roads. It is not mandatory to put the names of the residents on the mailbox, but this is certainly recommended.

What happens if you are not okay?

If your mailbox is not in order, the postman will inform you. If there is a serious problem that you do not correct, you risk having mail delivery suspended. More information about dimensions, regulations and possible problems and how to solve them can be found on the Bpost and PostNL sites.

Link from PostNL (mainly the Netherlands):

https://www.postnl.nl/ customer service/je-eigen-bandenbus/eisen

https://www.bpost.be/site/nl/verzenden/adresseringmijn-binnenbus