Open the Mail Settings page in the Mail section of the Account Settings:
Mail Storage Limit: This option is used to specify the maximum total size of the all Account Mailboxes. If a new incoming message cannot be stored in an Account, because the Account size would exceed the specified limit, the message is rejected and the message sender receives an error report. The current Mail Storage usage value is shown as a button, if the page is viewed by a System Administrator. By clicking this button you can re-calculate the usage storage counter if that counter was de-synchronized.
Mailbox Limit: This option is used to specify the maximum number of Mailboxes that can be created in this Account.
Archive Messages After, Delete Messages After: See the Chronos section for the details.
New Mailbox Format: This setting is displayed for multi-mailbox Accounts only. It specifies the default format for all new Mailboxes created in this Account.
Non-Mail Folders visible via IMAP: This setting controls if IMAP clients see non-Mail (Calendar, Contacts, etc.) Mailboxes. See the IMAP module section for more details.
Erase Deleted Messages: When this option is selected, the messages being deleted from the Account Mailboxes are rewritten with garbage first.
Encrypted Mailbox Creation: This setting specifies if the Account user is allowed to create Encrypted Mailboxes.