Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!

Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...

Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.

Etar - Material design open source calendar for Android.

eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.

Google Calendar - Spend less time managing your day & more time enjoying it