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…
Familiar is described a few different ways in the PHB, but the context seems generally the same down the board. I will provide a few (not all) quotes if I can find them.
Clairvoyance: “…in a location familiar to you (A place you have visited or seen before)…” -PHB pg 222
The entry for Scrying on PHB pg. 273 breaks down knowledge of a place or creature pretty well. Familiar: You know the target well
It boils down to there not being a written definition of what is or is not ‘familiar’ to your character. This means you may have to deliberate with your DM if you are uncertain. Are you familiar with your adventuring group? More than likely. Are you familiar with the Orc who carried you over the cliff into the ocean a few weeks ago? You might think so. Are you familiar with the necromancer you’ve only heard stories about and can pinpoint its exact location? No.
If I would have to make a ruling about familiarity, I would refer to the Scrying entry.