If you want one that has a calendar and supports inviting other people to events and accepting other people’s invitations - there’s really only Proton.
I tried mailbox.org, they use OX App Suite (Open-Xchange) on the backend which is full of holes with regards to calendar invites/reception support. I also tried Tuta which does not even bother to attempt trying to support accepting invites from other services (iCloud / exchange online / gmail).
If you want one that has a calendar and supports inviting other people to events and accepting other people’s invitations - there’s really only Proton.
I tried mailbox.org, they use OX App Suite (Open-Xchange) on the backend which is full of holes with regards to calendar invites/reception support. I also tried Tuta which does not even bother to attempt trying to support accepting invites from other services (iCloud / exchange online / gmail).