Mail. In the area I worked there was one apartment area with letterboxes at the door and about 20 areas with the mailbox in the doors of the apartments.

Sweden built a ton of housing (the goal was a million apartments in 10 years or so) back in the 50s and 60s. Those buildings didn't have letterboxes inside, all with letter gaps in the doors (at least where I worked). Newer places typically have them downstairs now.