A catch-all mailbox receives messages sent to non-existent addresses under the very same domain. For example, an email message sent to the miswritten firstname.lastname@example.org will be delivered to email@example.com if the catch-all feature has been activated for the latter. Thus, you can get messages from friends or customers who may have sent a message to your email address with a typo or to an obsolete one, which they may still have, but you have already disabled. Just 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 some moment you may start receiving spam emails in the catch-all mailbox and the forwarding restriction implies that the spam will not be re-sent to a 3rd-party mailbox.