One nice feature of cyrus IMAP is that it does support quotas. Quotas are applied on the basis of the mailbox and any sub-mailboxes that do not have quotas. This means that a mailbox can only have or be a part of one quota. When mail arrives the quota is checked to verify that the mail will not cause the quota to be exceeded, THE MAIL WILL STILL BE DELEVERED; however, the user will be notified  they are over the limit. If the mailbox is already over the limit when the mail arrives it will not be accepted.