> Any one email address refers to only one single person

This one hit me. My grandparents share a computer and one email address (just as they share one physical address and phone number), you wouldn't believe how many services, including Google, fails this rather simple test.

And in case you think this is a weird one: until not that long ago, every way to contact people where to the house they stayed in. Letters typically had a name, but if you were married and had shared accounts, either person could need to read those letters.