A catch-all email mailbox receives email messages sent to nonexistent addresses under the exact same domain. For instance, a message sent to the mistyped firstname.lastname@example.org will go to email@example.com in case the catch-all feature has been activated for the latter. Thus, you can get messages from friends or clients who may have sent a message to your email address with a typo or to an old one, which they may still have, but you have already disabled. Only one mailbox per domain can be a catch-all one and email forwarding cannot be enabled for such a mailbox. This is so because of the fact that at a certain point you may start receiving spam messages in the catch-all mailbox and the forwarding restriction implies that the spam will not be re-sent to a 3rd-party mailbox.