A catch-all mailbox receives emails sent to invalid email addresses under the same domain. For example, a message sent to the misspelled firstname.lastname@example.org will go to email@example.com if the catch-all feature has been activated for the latter. In this way, you can receive emails from friends or customers who may have sent a message to your email address with a spelling error 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 activated for such a mailbox. This is so because of the fact that at a certain point you may start receiving spam emails in the catch-all mailbox and the forwarding restriction implies that the spam will not be sent to a 3rd-party mailbox.