So for any developers interested in working on an open connector, I have some initial thoughts.
Before discovering Zimbra, we were using egroupware in combination with Kolab. It has a lot of holes, but at the time was a pretty solid mail solution.
Recently the egroupware dev community has released a couple options for an Outlook connector. Specifically I have noticed some great development on eGWOSync, an open GPL Outlook connecter. It syncs both calendar and address book with the egroupware solution.
I know there are other options such as the SF openconnector, but I thought these might be a great place to start. It is architected in pretty much the same way I think the Zimbra solution would need to be setup.
Outlook <-> client connector <-> XML-RPC <-> Server
So basically it would be mostly a matter of replacing the XML-RPC calls with SOAP calls to the Zimbra server... and then we're off
Have a look:
They have also done a lot of work on a SyncML implementation... so it might be agood source for that too.