Delete all mail from Inbox in Zimbra CLI
You can delete/clean mail's of user account when Hard drive (/opt) space get short . You may also take mailbox backup of these account before deleting external drive.
Login from Zimbra user
# su - zimbra
$ zmmailbox -z -m user@example.com emptyFolder /Inbox
For Junk Mail
$ zmmailbox -z -m user@example.com emptyFolder /Junk
You can also delete admin account mail's.
$ zmmailbox -z -m admin@example.com emptyFolder /Inbox