[RKDinOKC]

I manage both Exchange/Office365 and Kerio Connect mail servers and they both can share events outside their domain as long as both users are using Outlook. It sends a .ics calendar event file with the invite. Users don't try to share whole calendars, they usually just share an event and invite each other.

Matter of fact, years ago put up the Kerio Connect server so company users in Exchange could share events/appointments with their spouses if the spouses had an account in Kerio. Kerio Connect was less than half the price of an Exchange seat at the time. I host web/email for several small companies. I charge $5.95/mo per user for email. I am not a huge juggernaut like Microsoft so can't do $4/mo like MS does. My main difference is I offer superior antispam. But also backup everything, both on and off site, keep the software up-to-date, upgrade the mail server hardware every 3 years. And it helps pay for the internet connection. All my users really like it.