Zimbra is the calendar server.
If there is a recurring event (meeting) and I mark that I won’t be attending one instance of the future meeting on my PC’s thunderbird-lightning client and sync, Zimbra shows it correctly in the web interface (that I won’t be attending).
Afterwards, when I sync to the Android phone using DAVdroid, it does not mark it on the phone as “Attending: No”.
Android: KitKat (4.4.2)
Phone: Nexus 5