In order to keep disk and resource usage to a manageable level, your Zimbra administrator may enforce quotas to limit the amount of disk space your mailbox can consume.
The display banner under your name in Zimbra Web Client shows a bar showing how much of your quota you have used.
If you are over quota, the following things may happen:
You receive a notification.
Your mail interface may display a notification when you log in.
Mail sent to you may be returned (bounced), along with a notification that your mailbox is full.
To reduce the size of your mailbox, you can do the following:
Delete old messages that are no longer needed.
Look for messages with large file attachments and delete those. Save the file attachments locally if you want to preserve them.
Delete Documents notebooks, pages, images and attachments that are no longer needed.
Empty the Trash folder.
Zimbra also supports automatic purging of messages that are over a certain number of days old. Your email administrator sets the policy for when messages are purged, how old they have to be, and which folders are purged.