Retrochic's description is correct for European practice as well. Bednobs probably has no experience of that practice, which is not generally obviously visible in the UK because the post office (note, as in the old GPO) boxes, which actually exist, are (to my knowledge) hidden in a separate/back area ("private/elite", in genuine UK fashion) - in my locality they are (or at least were some years ago) served separately at a "special" counter - you took your ID there and the contents were brought to you (you signed for any mail requiring a signature and I think the collection was logged). I imagine that today these boxes continue to be operated in a similar fashion and that signed for mail continues to be delivered to them - have a look at this
http://www.royalmail.com/business/services/receiving/safety/po-box.
My "beef" is that when you hand an item for postal delivery anywhere one assumes it will be delivered through the postal network, including internationally. What Royal Mail in fact does is to sub-contract the overseas part to a commercial subsidiary of theirs which, unless I am wrong, is in fact no different to any courier service. I don't like the thought that equates to a letter on its way to me being handed by a Royal Mail depot to a bus driver going in my direction. The bus driver will take it to the nearest bus stop to me but then take it back as undeliverable because I was not standing there and live some 100m away.
A letter to a P.O. box abroad generally amounts to the destination address being the post office in question. The postal service in that country delivers to those P.O.Boxes all the time, nothing odd, perfectly routine. The point is that Royal Mail heaves the mail out of the postal service and puts it into the hands of a party who cannot deliver in the same way, nor can it deliver to any/all addresses that the postal service can and does on a daily basis.
Mamyalynne, I have and await a reply.