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Group Calendar and Email in a corporate setting with hundreds of people.

I guess gmail/google calendar can play in this space, but are there any open source options?

We're quite happy with what Linagora [1] offered a few years back. It's a French company so they work within EU framework of law. Their products are all open source, they only sell support services.

[1] : https://www.linagora.com/en/

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