Syncroton (Active-Sync) is working like a charme, access to my files via WebDav is also working.
After changing the webserver from +Apache to +Nginx the performance in using roundcube is now ok and after the logrotate during midnight I had no sync problems any more with my mobile.
Power consumption is below 3W so it costs only about 7€ per year (0,60€ per month) which is really cheap. Ok you have to add 1€ per month for the domain if you use +regfish for the domain.
A few days ago I exported all my contacts hosted on the google server and imported them into Kolab.
So if you don't like to let google know the phone number from your grandma and so on it's up to you to change this and get your own groupware on your server at your home :-)
BTW you can do a lot of other things with your Raspberry Pi.