All accounts can be assigned an account quota when created. This is the amount of space on the disk in megabytes that an account can use. Account quotas apply to email messages, attachments, calendar appointments, and Documents notebook pages. When the quota is reached, all email messages to that account are rejected. Users are not automatically notified when their quota is reached.
Users will need to delete mail messages, old calendar appointments, and old Documents pages off the server to get below their quota limit or you will need to increase the quota setting.
Users can view their account quota and percentage used in the header on the Zimbra Web Client. You can view mailbox quotas for all accounts from Monitoring>Server Statistics.
From the COS, to change the quota for all accounts associated with that COS, select the Advanced Options tab and change the Account quota field.
For an individual account, select the account to edit, click on the Advanced Options tab and change the Account quota field.