Here are instructions on how to setup an event that will notify you when you've reached or exceeded an email disk space limit:
1) Log into the email account via the SmarterMail web interface.
2) Click on the Settings (cog) icon at top.
3) Select Events on your left.
4) Click on the New button.
5) Give the event a name like "Email Full", make sure the Event Status is set to "Enabled", the Event Category to "User", and Event Type to "User Disk Space Used".
6) Click the "+ New Condition" button and select either Mailbox Usage (%) or Mailbox Usage (MB).
7) Select a Condition Operator, enter a Value, and click on Save.
8) Click on the "+ New Action" button and select "Send an email".
9) Customize the message you want sent and whom you want the message to be sent to. Click on the Save button.
10) Click on the Save button at top to complete the event notification setup.