[clug] Is there open source alternative to Office365 with Sharepoint?

[Stephen Hodgman]

G'day cluggers,
I have spent 20+ years pushing and using Open source applications myself
and for my clients to use in their daily work. 
So, please don't flame me for raising the topic of proprietary systems here.
My intention is to be able to offer a viable alternative to my customer
that uses open systems so they are not locked into one provider. 
Currently I have not found one so I hoped that this list would provide
some useful information.

I am currently stumped with how to propose an alternative to the
Office365 SMB offering being marketed here by Telstra.  I wanted to know
if anyone had an alternative offering for me to try. I will not bore you
here with the features of the product offered.

The key requirements are:
1. Group Email with integrated shared Calendars and Contact Sharing.
2. Desktop and browser based Email options.
3. Shared document storage in a cloud environment with local copies
("Cloud Drive" like Dropbox) to avoid downloads when editing.  This
drive needs to synchronise with cloud storage providing version control.
4. Cloud storage on a hosted server with backup etc.
5. Limited onsite infrastructure.  They will have MS Windows PCs(
necessary for specific applications)  with a Linux Domain Controller
with some shared storage that is local only. Most shared data to be
available "on the cloud"

They are currently looking to pay ~$16 per user per month for this service!
It seems to me that this is a market that Open Source should be able to
service.  I have not found anything.
e.g.How does one simply share  Calendars and Contacts with Thunderbird
while providing good integration with web & mobile apps? My experience
with using Google Calendars has been problematic over the years.

So, if anyone has any reasonable suggestions for an alternative provider
or setup I would be grateful.
I cannot be the only one with this requirement. Currently I am a little
frustrated.
Thanks for your tolerance.

Stephen Hodgman
0407 182 355
02 6236 6380