Well, no need for a delivery BOX, but we can deliver messages and items using our Retainers. There could be a new selection called 'Mail delivery' or something. A person types the player's name they want to deliver it to and your retainer delivers it to that person's retainer, who will in turn have a new 'mail' tab on the exchange items list. The retainer will also tell you that you have new mail when you summon it or talk to it in the bazaar.

Easier to implement it into retainers than to add mail boxes all around the city, and is more real.