I should recycle the catalogs that arrive in my mailbox every day instead of looking them intently for inspiration. Today I discovered this in the Pottery Barn Catalog:
Its a beautiful mailbox with a bird accent. This can be used as the card box and then hung up in front of your house!
Of course, you do not have to buy a new mailbox and re-purpose it for your wedding day: there are many vintage options available that can be incorporated into decor, used as a guest book or as a card box.
Original Cast-Iron Letter Boxes can often be found on Ebay or at Antique Fairs. I think they are very nostalgic and you can be sure that no one is going to run off with the cards!
British Postboxes have become such a popular option for Weddings in the UK that you can now purchase or "hire" smaller reproductions of the classic British pillar box! It comes in many colors but I would go with the iconic red color!
If the British post box is too extreme for your tastes, consider a simple and french-inspired wire mailbox: simple, delicate and also easily incorporated into your home after the big event!