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Posteo vs Proton Calendar: catalog facts
Posteo	Proton Calendar
Paid 1 euro per month, with paid upgrades for storage and aliases.	Freemium Free account available, with paid Proton plans and a business version.
Web	Android, iOS, Web
Signup without personal details Low fixed monthly price	End-to-end encryption of event details Built-in scheduling for booking meetings
No free tier 4 GB base storage	Requires a Proton account Works best within the Proton ecosystem
Web application, Hosted service	Downloadable app, Web application, Hosted service
Closed source	Closed source
License not stated	License not stated
Account 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.