Mail Drop and High Sierra
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I have just tried to use it for the first time in months and it doesn't work here. I tried to send a 15MB attachment from my iCloud address to another iCloud address; Mail didn't prompt me to use Mail Drop but just went ahead and sent it, so it was rejected for being too big (duh!).
I have Mail Drop enabled for my mail account (Mail Preferences) and Mail enabled for iCloud (in System Preferences) and haven't changed any settings since I don't remember when!