I've installed a Zimbra appliance version 184.108.40.2061 with 21 Gb of mail storage.
I have loaded in 4 mailbox a total of 5,4 Gb but the value in the status label of mail storage is 6/21GB mail storage free (28%).
I also prepare a default profile with 3 Gb
I don't undersand how Zimbra calculated this space value. thanks in advance