Zero Knowledge Encryption
Give users the option to keep their own encryption keys. Germany may have excellent privacy laws, but granting users ownership of the keys to their data would be ideal.

We’d be happy to implement this, but it’s impossible without breaking CardDAV/CalDAV compatibility. Once the standards support it: Sure. But until then: We don’t implement stuff that is just for show/marketing.
If you want details about how we keep you safe today, check:
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Adminfruux (CEO / Founder, fruux) commented
Posteo uses our core technology for their contact and calendar sync features. CardDAV and CalDAV don't support zero knowledge encryption without breaking the standard.
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Marjan commented
The german email provider "Posteo" is an excellent example to this topic:
https://posteo.de/en/site/encryption