It's great for multiple mail addresses. I get mail from multiple domains all in my ProtonMail inbox. Very easy to send from multiple addresses as well.
You can also turn on catch all addresses where you'll get all mail sent to anything @yourdomain.com. Very handy to create unique emails for sites I don't want to have my emails I actually use with contacts.
I can't really comment on multiple calendars. I haven't experimented much with that and get all my invites sent to one address.