A catch-all functionality that is activated for a given email mailbox will allow you to receive emails that are sent to addresses that do not actually exist. Otherwise said, if you deactivate an email address and someone still sends a message to it, or to any misspelled email address under the exact same domain, you will receive the message in the catch-all mailbox. Using this functionality means that you will no longer miss an email for any reason whatsoever, which could be vital in case you are running some business, for instance. Since you may start receiving junk messages, you can use the catch-all feature with anti-spam filters in order to get only legitimate messages in your inbox. Only one mailbox per domain name can be a catch-all one and email forwarding cannot be enabled for a catch-all mailbox.