Taking another person's mail could lead to some good spy RP. However, if it's a concern that it would become a problem, then we could use the current messenger NPCs and add a few with a system kind of like the storage one - right-click, 'check mail', and see if anyone's sent you a letter or package. To send mail, right-click, talk to the NPC, name the character, and trade them the book and/or item(s) you want to send them. Would that be hard to implement?