Protonmail is great but doesnt offer custom domains. If you need domains people mostly mention fastmail but i think there are far better choices like mailbox.org and kolabnow. Mailbox does not look like much from their homepage but it has awesome web client and it extremly reliable private provider thats in bussiness from 90s. I had account there for last 5 years without single problem.

This is actually not quite right, we have offered custom domain support since 2016 :)

https://protonmail.com/support/knowledge-base/custom-domain-...