What I have found so far:
- posteo looks good, but does not support custom domains
- tutanota can support it though I somewhat doubt their survivability long-term
- proton mail also supports it, but not a huge fan
- zoho: tried it years ago and it was ok, subprocesor list has aws and google on the periphery but not core
- ghandi.net: ?
- fastmail: ?
- kolab: ?
- Mailfence
Are there any others to look at?
I am not looking to self-host.
If privacy and security are your top priorities, I think hosting your own mail server is the way to go, preferably on bare metal. There are open source no-knowledge-encrypted mail servers you can compile and deploy, but the downside is availability, cost, and potentially speed.
It’s somewhat tempting, but don’t really want to spend a weekend debugging dkim keys or what have you if an important mail ends up in someone’s spam folder. Maybe for a project down the road.