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Proton Calendar vs Posteo: catalog facts
Proton Calendar	Posteo
Freemium Free account available, with paid Proton plans and a business version.	Paid 1 euro per month, with paid upgrades for storage and aliases.
Android, iOS, Web	Web
End-to-end encryption of event details Built-in scheduling for booking meetings	Signup without personal details Low fixed monthly price
Requires a Proton account Works best within the Proton ecosystem	No free tier 4 GB base storage
Downloadable app, Web application, Hosted service	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.