As the other person said, having a house or flat (apartment) door with a slot in it is probably the most common mail receiving method in Britain and maybe other European countries too. Ireland almost certainly, and common or not, I’ve definitely seen them in pictures of the Netherlands.

Some buildings might have a dedicated separate slot, but due to convention it’s often very near the door, and any external mail boxes aren’t far away either. They’re often on the wall next to the door.

I’m in California and the house I grew up in in the suburbs had a slot like that. I was kind of jealous of friends who had a “real” mailbox and going out to check the mail was a thing they got to do every day. I live out in the country now and the mailbox is 150 meters from the house so I only check it once per week. If there’s a package that’s too big to go in the box the letter carrier has to drive all the way up to the house and gets pretty pissy about it, they definitely would not come shove letters through the door every day.