You can self-host it. I worked there briefly. Their market is big enterprises and govt that won't fly a hosted solution, they offer both a SaaS and self-hosted

You can self-host it. I worked there briefly. Their market is big enterprises and govt that won't fly a hosted solution, they offer both a SaaS and self-hosted licenses Then it's up to you how you sync with calendars, if a self-host caldav instance is enough no microsoft there