> How can you know that it "works"? Any company scummy enough to send spam to begin with, is capable of selling their customer data to a network of scummy companies that will do the same thing.

That’s quite a stretch for a company sending marketing email with a broken unsub mechanism.

Considering how these companies are infamous for making it difficult to unsubscribe from their service in real life, I don't think it's too much of a stretch to attribute malice to how they conduct email communications.