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.
I use Zoho. Although it’s not as secure or private as Protonmail or Tutanota, it’s pretty decent, has great features, and most importantly, is affordable. The price is $1 per account, which is much better than Protonmail’s starting $4 price, for example. Zoho has many features and a great control panel, which makes managing many accounts for an organization a breeze.