I am probably repeating a post here, apologies if so
We currently use a Linux server with postfix and a bespoke company calendar.
I have ZIMBRA setup as an E-mail server and company calendar. First impressions are that this is the way to go. Easy to manage and use!!!
I have worked out how to view shared ZIMBRA calendars in Outlook 2013 (took a bit of working out but works well now) and how to set the update frequency.
Unfortunately, you can't directly set an appointment up via the shared calendar. For example, if I want to set up a meeting with my Director, I can E-mail the Director a meeting request, copy in firstname.lastname@example.org and the shared email@example.com calendar will update in outlook as will the firstname.lastname@example.org calendar. EXCELLENT.
Unfortunately, even if I create the new meeting from my ZIMBRA E-mail address in outlook, unless I copy myself in, only my Outlook Calendar (This computer only) updates.
I realise this is Open Source and not collaboration so not expecting the same features collaboration has but there must be a way to share my outlook calendar with ZIMBRA or share my ZIMBRA calendar as read/ write.
Apart from this, ZIMBRA is a great system.