What should be written on the mailbox or by the letter slot?

All mail recipients at the address must be written on the mailbox or by the letter slot. This also applies if there are children or lodgers living at the address.

You must at least state your last name and initials, e.g. "P. Jensen". If you live in an apartment building, you must also state the floor, side, room or apartment number.

You must ensure that you use self-adhesive letters or nameplates of a quality that will not fade or disappear due to weather or vandalism.

(from the Executive Order on Postal Services and Postal Transport)