According to Cambridge dictionary, in US, a mailbox is a box outside a person's house where letters are put, but in UK, it's a box in the street or other public place in which you can put letters to be collected and sent.
The dictionary states that postbox has the same meaning as the second meaning of mailbox (a public box) without mentioning US or UK usages. I could also find the word letterbox in the dictionary which seems to be used mostly in UK meaning both a mail slot and a postbox. According to the dictionary, mail slot is used in US to mean a rectangular hole in the door or in a wall near the entrance of a house or other building, through which letters, etc. are delivered. As a non-native speaker of English, I have these questions.
1. Can I use postbox safely to mean a public box for letters in UK, US and Australia?
2. What word(s) can be used in UK to mean the private box for letter outside your house?
3. Is there any word/words that you can use to mean the private box for letter outside your house without ambiguity in UK, US and Australia?
4. Can I also use mail slot in UK to mean letterbox?
5. Do you have any other suggestions?
Best Answer
I can only answer from a British viewpoint, but I think you are falling into the common trap of thinking "every country has the same things, they just use different words", which has never been true.
Post box or postbox is the common term for the boxes used by the Post Office to receive letters for delivery. Many of them are freestanding and circular, and are called pillarboxes, which is sometimes used by metonymy to refer to all Post Office boxes.
A box outside your house to receive letters is extremely rare in Britain; where they exist, they might be called mailboxes by analogy with America. Most houses have a slot in the front door for letters, sometimes with a plywood box or wire cage inside to catch the letters before they fall on the mat. This (whether or not it includes a box) is called a letterbox. Mail slot would be understood to refer to the slot. but is very American.