The Google Apps Email Storage!
If you approach your storage limit, the quota indicator will change from green to red. If you go over your limit, incoming messages to your account will be returned to the sender.
If you've hit your limit and need to clear space, you'll need to move messages to Trash and permanently delete them from there.
To find messages that are larger in size due to attachments, try using advanced search operators such as has:attachment or filename:mov.
To move a message to Trash: To permanently delete a specific message: To permanently delete all of the messages in your Trash at once: Messages left in Trash for 30 days will be permanently deleted automatically. Messages that are permanently deleted, manually or automatically, cannot be recovered.