A catch-all email mailbox receives messages sent to invalid email addresses under the very same domain name. For example, an email sent to the miswritten firstname.lastname@example.org will go to email@example.com if the catch-all feature has been activated for the latter. Thus, you can receive emails from mates 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 deleted. Only one single mailbox per domain can be a catch-all one and email forwarding cannot be enabled for such a mailbox. The latter is due to the fact that at some point you may start receiving spam messages in the catch-all mailbox and the forwarding restriction means that the spam will not be delivered to a 3rd-party mailbox.