cPanel email's Default Addresses are also known as "catch-alls." With a Default Address enabled, cPanel routes any email sent to addresses at your domain name that do not exist to the account's Default Address. For example, if your Default Address is firstname.lastname@example.org and someone emails a non-existent address, email@example.com, cPanel forwards the email to firstname.lastname@example.org.
|Discard with error to sender||Enter an error message when an email is sent to an invalid address.|
|Forward to email address||Enter the address where you want to forward emails sent to invalid addresses.|
You can also use one of the Advanced Options, which cPanel has documented here.