The catch-all function is an option which you can disable (default) or enable. It is entirely your preference. You can enable or disable this function by clicking the Logon to Mail Web Interface As Asministrator link in Email Manager section of control panel or log on to firstname.lastname@example.org.
In short, the catch-all email function allows you to receive all incoming email that is addressed to non-existent email boxes. You can assign one of your POP boxes to capture all incoming emails destined to email addresses associated with your domain which has not been set up as an email address (eg POP, Forward, or Autoresponder).
If someone sends email message to an email address that is not setup as a POP, forward, or autoresponder, then the catch-all email function can forward these emails to an existing POP account which you specify. Therefore, you can "catch" all of the email that is sent to your domain name.
For example: A user may send emails to non-existent accounts if there is a typo in the username. e.g., instead of sale@YourHostedDomainName.com the user mistakenly sent an email to sael@YourHostedDomainName.com. Instead of bouncing the email, your catch-all function will allow you to capture this email message.
If the catch-all function is disabled, then any incoming email addressed to non-existent email accounts will be bounced back to the sender.