I don't trust them with that. They have an extremely limited free tier that indirectly "forces" users to upgrade to one of the paid tiers. The one that is least limited and reasonably priced is "Proton Unlimited" for ten bucks a month to be paid annually. Plus: you cannot even use a mail client of your choice without installing an extra application for that.

You could easily selfhosting the cloud stuff. If you don't want to selfhost a mail server you could use one from a mail provider. Don't know how it's done internationally, but here in Germany are quite a few companies that provide you with either a domain forwarding service for your selfhosted stuff, or a good and cheap mail server solution for a fraction of what Proton wants to have, and I highly doubt they need ca. $120 a year per user to offer a handful of mail aliasev and allowing you to create folders in your mailbox.

And you're not even arbitrarily limited - and your data never leaves your private environment.