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Microsoft Outlook vs Proton Calendar: catalog facts
Microsoft Outlook	Proton Calendar
Freemium Free app and account available; Microsoft 365 plans add paid features and Office apps.	Freemium Free account available, with paid Proton plans and a business version.
Windows, macOS, Android, iOS, Web	Android, iOS, Web
Email, calendar and contacts for multiple accounts in one app Available on desktop, mobile and web	End-to-end encryption of event details Built-in scheduling for booking meetings
Closed source and tied to Microsoft services Some features, including Copilot, require a paid plan	Requires a Proton account Works best within the Proton ecosystem
Downloadable app, Web application	Downloadable app, Web application, Hosted service
Closed source	Closed source
License not stated	License not stated
Not stated if an account is needed	Account needed
Not stated if it works offline	Not stated if it works offline
Official website Full listing and sources Checked September 23, 2026	Official website Full listing and sources Checked September 23, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.