By configuring the Catch-all section, you can decide how to treat emails sent to non existing users in this domain. The available options in the selection box are to have them rejected, to redirect them to one of the existing public folders or to redirect them to a catch-all account. If you choose to have the emails redirected to a public folder, make sure you create that public folder first, so you can pick it from the list. If the emails are redirected to an account, you can also specify a folder for the emails to be stored in, using the "Change folder" button.