When a user creates a mailbox, the mailbox is created on the IMail Server system. Because the IMail Server will be the permanent storage location for IMAP4 users' mail, you need to configure the server with appropriate disk space and manage the disk space by monitoring mailbox disk usage.
You can set maximum mailbox size and maximum number of messages for each user or you can set the maximum mailbox size and maximum number of messages globally for all users on a selected e-mail domain :
Administrators can set an option (on the IMAP4 tab) that determines whether users must subscribe to a private mailbox before they can read it.