The ability to see up sieve filtering is great too. I’ve got a massive script that automatically sorts and files away most of my emails.

I want to like Proton Mail, but their sieve filtering kind of sucks, and with large mailboxes it slows down to an almost unusable amount.

Fastmail is awesome. If you want to set it up as receive only, you can set up CloudFlare email forwarding for free and have it forward to your regular account.