Open-source alternatives to Outlook

Beautiful open source mail client for desktop — :love_letter: A beautiful, fast and fully open source mail client for Mac, Windows and Linux.

↔ alternative to Apple Mail、Outlook

End-to-end encrypted email with calendar and contacts — Tuta is an email service with a strong focus on security and privacy that lets you encrypt emails, contacts and calendar entries on all your devices.

↔ alternative to Gmail、Outlook、ProtonMail

Mail and calendar client for Windows — Built-in Mail & Calendars app clone for Windows.

↔ alternative to Outlook