A catch-all mailbox receives email messages sent to invalid addresses under the very same domain. For instance, a message sent to the misspelled 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 emails from acquaintances or clients 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 disabled. Just one single mailbox per domain name 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 a certain moment you may start receiving spam email messages in the catch-all mailbox and the forwarding restriction means that the spam will not be delivered to a third-party mailbox.