In ZCS 7.x dumpster folder is default disabled.
The settings are part of the COS:
su - zimbra
# zmprov getCos default|grep -i dumpster
zimbraDumpsterEnabled: TRUE # it must be TRUE value
zimbraMailDumpsterLifetime: 30d # (mail / briefcase / calendar, etc)
In my opinion Dumpster folder (feature) check the user quota value, because the dumbster items are still in /opt/zimbra/store/ user folder (I'm not 100% sure because I don't use current quota value for my users) - this is easy to check it, if you have enabled quota feature.
Dumpster feature works like the next level trash folder.
It' s best practice for me is changing the value zimbraMailDumpsterLifetime max 30 days to max 7 days.
Please read this post also:
[SOLVED] zimbra 7.0 dumpster