Yeah. It always shocks me at how cheaply these people offer this information. Even going back to the Cold War, Aldrich Ames was selling info that directly led to peoples’ deaths for just 10s of thousands of dollars. Nowadays people are selling details of billion dollar ships for less than the price of a new car.
Security is hard
I would say white-list instead of black-list if possible.
Beside the point, have you considered the reasons why you might not want to run your own email server in todays age? It’s a fun experience for sure, but if you want it for serious use it’s not for the faint of heart (unfortunately).
Edit: also lol to your friend unknowingly conducting a spam campaign from your server