In my experience the amount of spam is directly related to the domain (and what it's digital footprint is).

That being said, for my hobby/personal domains, the spam has been as close to zero as you could reasonably expect (maybe a few per week, and they are correctly flagged as spam).

I see less spam in my inbox (effectively 0) than I did with Gmail, but that could be because I've been more careful about what I sign up for than I was with my old email (which I got as a kid).