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Drive-thru postal delivery is not a known phenomenon in the UK. The steering wheel is in the right place, and the postman or postwoman must alight from his or her vehicle to deliver the post. This approach allows postal workers to discuss the weather and partake in tea-drinking ceremonies that a fundamental part of postal deliveries in this country.
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People here often get to know their postal carrier very well over the years. There's an off-color joke about a gift for a retiring mailman, but I won't go there right now. The punchline, however, is: "The lunch was my idea."
In cities, towns, and other high-density areas, mail is delivered on foot, door-to-door. The door at my parents' house had a mail slot in it for delivery, but when they replaced the door many years ago they added a mailbox (letterbox) outside, on the porch, so the mailman still had to come in the yard to deliver it. Small apartment buildings usually have a set of mailboxes by the front door, each with their own key, with a master key for the postman that opens the entire front so he (she) can insert the mail in all the boxes at once. Larger complexes have similar, although larger, sets of boxes.
In rural areas, where houses are spread farther apart and often far from the road (our house is about 140 feet from the road on a 200-foot deep lot), mailboxes are set at the curb or side of the road. Depending on the route, they may be clustered in one area, and they may also be on the opposite side of the road from the house, so the carrier need only travel in one direction to deliver the mail. Jokes aside, the Postal Service does strive for efficiency where possible.
I've seen one house that has two mailboxes, one at car door height and one about 10 or 20 feet in the air. That one is labeld "Air Mail."
The "wee flag" on the mailbox is to alert the carrier that there is a letter for pickup, i. e., to be mailed (posted), saving the homeowner a trip the to post office, which can be a hoot and a holler away in some areas of the country.
(If there's no mail for that address he will pass right by the box if the flag isn't up.) If you don't have any stamps, you can leave an envelope with money and a list of what you want and you'll find stamps and a receipt when you come home.
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My sister's new house has a letterbox in the front door (that's where one's post is delivered — into your house), but, as one has to go through a lockable side gate to get to the door, her husband has placed a mailbox (note US name) outside the gate. She feels somehow detached from her postal service because of this.
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Ah, such is the price of security, even in Jolly Olde . . .
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In the US, if one posts (whoops: mails) a letter in a mailbox, that is blue without a red top, and receives letters in a mailbox, with one's name and a wee flag on it, how does one distinguish between the two types of mailbox? Of course, here one posts to a postbox, and, in the unfortunate situation of my sister, receives in a mailbox, and, in the usual situation, receives in a letterbox.
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What's to distinguish? Since we speak English, we have a plethora of words available to name or describe our mailboxes.
The big blue corner boxes are for depositing letters and packages with sufficient postage applied to be mailed/posted. The small one at your house is for receiving mail and packages (at least, packages that will fit in the box; if it's too large, or if a signature is required for delivery (e. g., Registered Mail), you get a notice to come pick up and/or sign for the mail at the local post office. You can always leave the notice in your box with the flag up and write down the date you will be home to recieve and/or sign.