I just moved to Prague and my new house has the letter boxes in the hallway, meaning that in order to throw letters in you have to enter through the main door which is usually locked. I did receive a letter before, so the post officer seems to be able to get in somehow. How does this work here? Does the post get keys to all the houses or is there a person in charge of letting the post officer in every day (there is no reception)? Will UPS and similar services be able to throw in we-could-not-deliver-your-package-notes?
This will probably differ from city to city or post office organisation.
In the past (in Germany), when there was only one postal service, they did recieve keys for each house on their route. When the letters were picked up at the local post office, they would take the (often gigantic) key ring with them and return it the post office later for storage.
When alternative postal services started, they generally didn't recieve keys and they had to ring someone to get in.
Today, after many of the local post offices have been closed, the letters are often stored in local storages on the street. I have noticed that they often ring to get in, so I assume they no longer have the key rings since the local storages are not very secure.
I just met our deliverer and asked about this. She confirmed that they (German Post), but also other postal services, receives the keys for most houses. Some houses, however, are not cooperative and they must ring to get in. If this is not possible, they report this. (I did not ask where the keys are stored)