Since there is explicit language for the case the target doesn't know the caster despite the familiarity requirement, I'd allow the Margo Robbie situation.
If you're a player, easy, ask your DM.
If you're the DM, tough luck. Go on an Internet forum and hope nobody tell you to ask your DM
You'll have to determine what level of information the character must acquire to be familiar with someone without the target necessarily knowing the sender. Is one familiar with all his coworkers outside immediate collegues ? Is having met enough ? Is the character familiar with a mere employer?
You'll also have to determine if the sender knows intuitively when he's familiar enough for the purpose of the spell (so he doesn't lose a spell slot) or if you prefer him to cast it in the wild without knowing (unless an answer is given there is no way to know whether the sending got through) of his secret admirer routine/careful study of the target was enough.
From what I have seen the players mostly use it for intra party communication (so familiarity can be assumed) but it might be used for emergency communication. If you declare that the target is unsuitably familiar without havibg a clear idea of why, you might pass for this kind of DM that doesn't design adventures to take PC powers into account and resort to nerfing them. And you risk to teach the players to develop a sending-routine of testing sending with potential contacts while they are safe to ensure communication later when they will need it, if they have been burned once. Which isn't very fun.
I'd err on the side of player empowerment and give them a lot of leeway, not introduce "loss of familiarity" (are you still familiar with your old childhood friend you have not seen in 20 year?) and generally let the spell work but
the target could use its 25 word to answer "who are you? what, you creep, you've been studying me?!? STOP THAT!"
And don't count words too strictly either. Insisting on exactly 25 will just have the group hunt for synonyms and convoluted writing style and since the target doesn't need a common language with the author you'll soon be faced with the question: is draconic an agglutinative language?