Actually, the one-to-one emails were only a small part of their business model. Earn has become popular among crypto companies for doing paid mass email campaigns.

If you are on the site as a user it's obvious. You get a lot of paid messages if you sign up for the right lists. Not sure if it will scale, but interesting use of crypto.

This is kind of a silly site, but it's one of the few articles I can find on the topic. https://sludgefeed.com/earn-com-becoming-go-to-crypto-advert...