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?