A catch-all mailbox receives email messages sent to invalid email addresses under the very same domain. For example, a message sent to the miswritten firstname.lastname@example.org will go to email@example.com in case the catch-all feature has been enabled for the latter. Thus, you can get email messages from acquaintances or clients who may have sent a message to your email address with a typographical error or to an out-of-date one, which they may still have, but you have already deactivated. Just one 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 point you may start receiving spam email messages in the catch-all mailbox and the forwarding restriction implies that the spam will not be redirected to a 3rd-party mailbox.