Please note that we currently don't reject new emails going to mailboxes that are over quota.
A mailbox quota allows you to limit the amount of disk space that a mailbox account can occupy. Assigning a quota of 0 MB imposes no limit on the storage space an account can take up. You can assign a quota to a mailbox when creating it or any time after using the manage mailbox page of our Control Panel.
We currently do not send automatic notifications for near or over quota mailboxes but we have a PHP script available that will let you know which email accounts are near or over quota limits. This is availalble from our public GitHub repository here