BUG: new messages to person that is not sharing

I propose a solution for bug #3016, just check if person is sharing.
Please don't use single letter variable names, I think typing `recipient` two more times is okay ;). Also you can use `Person#local?` to do that check, I think it reads nicer and it provides a single place to change if we ever need to change it: `if recipient.local?`. Finally since this (rightfully) only validates local recipients, how about naming it `local_recipients`? :)