Groupware/Calendars - an open alternative?

Question

Calendar

Hello hive mind.
It is necessary, ideally, an open / OpenSource analogue of Exchange, but this does not exist in nature.
As a living wage, you need a server opensource calendar synchronizer for a whole fleet of Outlooks. 2003, 2007, 2010 - there are all sorts. Just do not touch the mail. You can also contact.
If there is no such thing in nature, then what would you recommend from reliable in the sense of groupware for the web? Because my eyes widen from several systems and tons of functionality they offer, but firstly, synchronization of calendars (access levels, editing, groups, etc.) is mainly needed, and secondly, contacts and notes, no more.

Answer 1

Try to look towards the zimbra .

Answer 2

phpgroupware, egroupware

Answer 3

https://www.altlinux.org/SOGO