What's to stop spammers from just making up identities? "people you don't know" would would work for whitelisting (i.e. people I don't know wouldn't get whitel

What's to stop spammers from just making up identities? "people you don't know" would would work for whitelisting (i.e. people I don't know wouldn't get whitelisted), but as I need and want to receive the occasional email from random strangers, it's not all that useful a criterion for marking something as certainly spam.