As I said in another thread, there's really zero need to implement a mailbox system for item deliveries - it should be extremely simple to build that functionality into retainers. All you would need would be an extra tab in inventory management that holds items sent to you, complete with message, as well as a a button to send to another player.

I much prefer building onto existing systems and functionality instead of shoehorning in entirely new systems simply because that's how other games do it.