@Igor Ok, very good.

(13) Regarding your response to (12), there must be some way to keep track of when an accident happens, right? What attributes would you like to store about such situation?

(14) So, when the person addressed in a letter picks it up from the destination warehouse the process comes to some sort of end, right?

@Igor Yes, that's what I have considered before, as noted in the previous comments, but such point certainly requires a more detailed analysis, and I am making these series of questions in order to define the situation more clearly.

Just to clarify: Is this database going to be implemented to be used in a real world business context, is it for homework, or some sort of personal project (which kind?)? I wish to continue helping as much as possible, but I want to know the actual scope of this endeavour.

(15) Should the person addressed pick a letter up always directly from the destination warehouse or it can optionally be delivered to some kind of specified location?

(16) At what specific point would a tentative delivery attempt take place in the whole process, for example, in the middle, at the very end, etc.?