To create a catch-all address for delivering messages addressed to non-existent recipients:
Sign in to the Google Admin console.
Click Google Apps > Gmail > User settings.
In the \Email routing section, select the Route to catch-all address: radio button.
Enter the user name to which mail addressed to unknown users will be forwarded. The user name must be a valid account in your domain. Do not include the domain name; Google Apps automatically adds @your-domain to the address.
Click Save changes.
To bounce messages addressed to non-existent recipients:
Sign in to the Google Admin console.
Click Google Apps > Gmail > User settings. Where is it?
In the Email routing section, select the Discard radio button.
Click Save changes.