The documentation says that client mailbox quota specifies the max. hard drive space (in MB) that this client's email accounts can use. -1 means unlimited. The question is how it is forced? Is it limited by the filesystem quota ( I do not think as all the files owned by the vmail user) or calculated by the IMAP server/maildrop somehow? Or it is used only by ISPConfig not allowing to set sum of individual mailbox quotas greater than client mailbox quota? I do understand that mailbox quota for individual mailboxes checked by maildrop and it works as expected.