Outlook with email, calendar & contact sync
https://sourceforge.net/projects/outlookcaldavsynchronizer/
is an Outlook plugin that will sync with gmail or any other calDAV server. eg:
https://www.cyrusimap.org/ (Email, contacts & calendar server)
http://radicale.org/
https://www.davical.org/
https://sogo.nu/
https://www.calendarserver.org/ (Apple Darwin)
https://www.horde.org/