A catch-all mailbox receives messages sent to invalid email addresses under the exact same domain. For example, an email sent to the miswritten email@example.com will be delivered to firstname.lastname@example.org in case the catch-all feature has been activated for the latter. In this way, you can get emails from acquaintances or customers who may have sent a message to your email address with a spelling mistake or to an old one, which they may still have, but you have already disabled. Only one single mailbox per domain can be a catch-all one and email forwarding cannot be set up for such a mailbox. The latter is due to the fact that at some moment you may begin receiving spam messages in the catch-all mailbox and the forwarding restriction means that the spam will not be re-sent to a 3rd-party mailbox.