honestly… There will be no one giving you any warranty. Felix already mentioned that they’re not testing anything regarding the old legacy client anymore. On other hand they’re not breaking compatibility with newer versions of kopano core currently. Outlook 2003 is already far beyond being any point of suppport.
Actually with Outlook 2003 you should not experience any update troubles like with Outlook 2010 or 2013 since there are no updates at all anymore.
From the Kopano Core 8.6.7 version all I can tell you it that everything I need is working with Outlook 2010.
I actually still use Outlook 2010 and will use it as long as it works. Either I will have time to finish some project regarding Mapi over HTTP (but other things have more priority currently than that like i.e. family inc. children) or I will change to DeskApp until I have a working Mapi over HTTP interface. So I’m at least being prepared for some change in kopano core that might break compability to old Zarafa Client. Don’t know what you mean with calender only synchronisation but if it is meant in context to have calendar details of all employees available on the road - maybe you should have a look at latest 2.4.4 Z-Push and use some iPad / Android Tab for this purpose… There are changes in 2.4.4. regarding making shared stores available by using formatted user names for login to shared stores.