Nextcloud Calendar - https://apps.nextcloud.com/apps/calendar

Radicale - https://radicale.org/

SabreDAV - http://sabre.io/

SOGo - https://sogo.nu/

Mailcow - https://mailcow.email/

Citadel - http://www.citadel.org/

Agendav - https://www.agendav.org/

Kopano - https://kopano.com/