When using a catch-all address it would be useful to block certain addresses and return a “550” response (from the SMTP RFC: 550 Requested action not taken: mailbox unavailable (e.g., mailbox
not found, no access, or command rejected for policy reasons).
Sometimes an address becomes widely distributed on SPAM lists and is targeted in high volume. Instead of just silently accepting and then dropping email to the address, a negative / invalid response serves to notify the sender that they are attempting to send to an invalid email address.
Large numbers of invalid delivery attempts can be be used by upstream mail delivery providers to detect spammers. It will also tend to make spammers remove the invalid address from their lists, thus reducing useless (accept and drop) and energy wasting email traffic.