Who wants this?
It is simple.
Have a custom domain, if you want. Or else, existing domains offered by Fastmail work fine.
Have an alias for each website. For example, if you order pizzas, have one for that. For a travel website, have another. The trick is NOT to give out your actual email id but give the alias for that particular site.
This is how it plays out. Go to dominos and have an alias like email@example.com (or whatever domain you want). It will immediately segregate your email. If you are spammed for that domain, it is a matter of deleting that alias. Simple. Quick. Painless.
I have folders for all incoming mail, and Fastmail allows setting up rules to sort them out automatically. For example, if I have a newsletter subscription, it is set to flow in that folder and marked as read. Or anything else that I wish to read later.
Achieve that today!