The best open source Calendar alternative to Zoho Mail is Proton Mail. If that doesn't suit you, our users have ranked more than 100 alternatives to Zoho Mail and six of them are open source Calendar Apps so hopefully you can find a suitable replacement. Other interesting open source Calendar App alternatives to Zoho Mail are Tuta Mail, Posteo, Infomaniak kMail and Kolab Now.
Proton Mail provides secure email services with end-to-end encryption, ensuring data protection. Swiss-based and outside US jurisdiction, it offers a free version to champion privacy as a human right. Fully accessible on any device.
Encryption and secure service.
Offers comprehensive encryption for email, calendar, and contacts with zero personal info required. Built on open source, install via Play Store, F-Droid, or APK, with Tor and VPN support. Both free and paid plans; utilizes quantum-resistant encryption ensuring privacy-focused, anonymous usage.
Free plan does not allow your own domain
Its open source, so its bugs will always be minimal.
Web- and IMAP-based email service. 4GB cost 1€/month. Information is hosted on encrypted hard drives. Electricity from renewable resources is used.
Privacy-focused Swiss email system provides 20 GB free storage, supports custom domains and unlimited storage, syncs contacts and calendars, is ad-free, and is part of a secure productivity suite used by over a million users across Europe on all devices.
Kolab Now provides a suite of intuitive apps in a user friendly environment to make your day to day online comms easier, smarter and most importantly; secure.
Afterlogic Aurora is an enterprise collaboration system for small and medium-sized business. Aurora provides collaboration of webmail, contacts, calendar, files and helpdesk modules.
Switzerland
Germany
EU
United States
Free plan does not allow your own domain