Design: When the line between domain objects and service objects isn't clear?

You're missing something: it's completely sensible for the Mailbox object to depend on an interface that's provided at runtime. The "don't depend on runtime services" is correct, in that you shouldn't have compile-time dependencies.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.