From this thread, I've also started looking into Kopano, a bit of information for others looking - the website you want is "kopano.io" for the open source version, this isn't mentioned very well on their commercial website.
With that said, It's actually a fairly interesting alternative.
Officially they only offer nightly builds (For free), but if like me that sounds like a crazy idea for a production environment... there is Debian as an option, as it appears to be included in the Debian repositories:https://packages.debian.org/search?lang ... rds=kopano
This means you can install simply using "apt-get install kopano-core", and it allows using the system default versions of postfix, clamav etc. And of course, a non-nightly stable version... this is actually sounding very promising.
I sometimes disliked how with Zimbra you couldn't easily update a single component such as ClamAV without some big hacks
And the open source version has an official supported backup process
Although, there are a few missing things such as information about enabling DKIM etc. But it does appear to be a viable option, but with a lot more work necessary to get it running, and a somewhat lack of documentation.
However, being included in a distributions official repository is a good sign towards the open source side of things. So Kopano has jumped to the top of my "possible" alternatives list.
Anyhow, just my notes so far for anyone interested