zmmailbox is a Zimbra admin command for mailboxes
Zmmailbox - Zimbra :: Wiki
curl is a command line tool for transferring files with URL syntax
We just want to know if there is some permission problem.
As far as why the zdb grows, it could be all the local failures we're putting out. We fixed a bug (39687) that detects the over quota condition and prevents repeated failures. We undoubtedly should do the same for this.
If I could find the exact repro steps for your problem, we could fix this.