The Sending spell states that:
You send a short message of twenty-five words or less to a creature with which you are familiar. The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately. (PHB, p. 274.)
So what counts as 'familiar'? I initially assumed that the intent was somebody you personally knew, perhaps by name, but the inclusion of the phrase "if it knows you" would suggest that the target of the spell need not have any idea of your existence.
Is passing somebody on the street enough? Seeing them from a distance? Being introduced once? How about having them described to you by somebody who does know them?
I'm wondering, among other things, what implications this has for magical spam mail...