If your domain suddenly stopped receiving email or someone tried to email you and the message was returned with "Domain Size Limit Exceeded", it likely means your email domain has stored the maximum amount of email data on the server it can currently hold, otherwise known as reached it's disk quota.
The default amount of email storage is 1000MB per domain to be shared among all mailbox users. Additional email storage can be purchased in increments of 1000MB at the rate of $4.95 per month, billed according to the account's billing cycle (Monthly or Annually).
You can review your email disk consumption from the Reports section of the Smartermail web interface. When logged in as an email domain Administrator, you can see a Reports icon on the left side navigation. Click there, then use the sub menu navigation pane to find and expand the sections labeled Domain Summary Reports -> Traffic Reports -> Disk Usage. The main window will load with a graphical itemization of the disk usage, including a list of the top email disk space consumers.
You can click any one of the mailbox users to get an itemization of their mailbox folders and see whether or not they have an excessive amount of consumption in Deleted Items or Junk Mail which often consumes disk space unnecessarily. Removing items from these folders will immediately reclaim disk space for your email domain.