Calendar and Contact Sync
Calendars and contacts contain some of your most sensitive data; use products that implement E2EE at rest to prevent a provider from reading them.
Tutanota offers a free and encrypted calendar across their supported platforms. Features include: automatic E2EE of all data, sharing features, import/export functionality, multi-factor authentication, and more.
Multiple calendars and extended sharing functionality is limited to paid subscribers.
Proton Calendar is an encrypted calendar service available to Proton members via web or mobile clients. Features include: automatic E2EE of all data, sharing features, import/export functionality, and more. Those on the free tier get access to a single calendar, whereas paid subscribers can create up to 20 calendars. Extended sharing functionality is also limited to paid subscribers.
Proton Mail can be used to synchronize contacts. Likewise, the service is currently only available via the web and mobile clients.
Proton does not use E2EE for your contact names and email addresses.