A catch-all mailbox receives emails sent to invalid email addresses under the same domain. For example, an email message sent to the mistyped suport@domain.com will go to support@domain.com in case the catch-all feature has been enabled for the latter. Thus, you can get email messages from buddies or clients 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 deactivated. Only one 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 messages in the catch-all mailbox and the forwarding limitation implies that the spam will not be re-sent to a 3rd-party mailbox.