When mail is sent in to your site it is checked against the names and aliases of the users set up for you, if it can't match anyone it refuses and bounces the mail.

One of the users can be set up as a catch all account which means that any mail which would have been bounced will now be delivered to that user.

To set up a catch all account enter '@www.example.com' as an alias for one of your users where www.example.com is your full website address.