To redirect a message from its original recipient to another, but only if the account quota was exceeded, perform the following:

Log into the webadmin interface.

Go to the "Security & Filtering" -> "Acceptance & Routing" context.

Go to the "Advanced Settings" tab.

Click the "Add Acceptance / Routing Rule" button.

Enter a descriptive name in the "Rule name" field.

From the "Conditions" section, select "Recipient -> Is Overquota".

Click "Add condition".

From the "Actions" section, select "SMTP->Action"

Click "Add action".

Set the action "SMTP action" to "Accept" and add an optional message

 Again, from the "Actions" section, select "Recipients -> Change Recipient".

 Click "Add action".

Enter a recipient name and destination folder.

Click "Save Configuration".

NOTE: The destination account must exist. If it does not, the redirection will fail and the message will still be discarded.