Earlier quoted context omitted.
I've tried it, but their solution is (IMHO) a very poor fit for a business e-mail account, as there is (or was when I used the service) no way to manage e-mail accounts for your employees and no way to archive / extract their e-mails in case of need (I understand that it's a privacy-focused e-mail service, but as an employer you have legal requirements to keep business documents for several years, and having to rely…
It looks like ProtonMail released new account management features just a few weeks ago, including the capability for "organization admins to access the emails of other organization users" while still preserving end-to-end encryption. Source: https://protonmail.com/blog/encrypted-email-for-organization...
I wonder how can they provide this without either storing encryption keys on their servers or reencrypting (client-side!) all email during such events.