A Catch All account is a default mailbox where all mail that isn't destined for a specific mailbox goes. For example, if somone sends an e-mail to sales@yourdomain.com but you haven't set up a mailbox called sales then the e-mail will go to the catch-all mailbox.
You can set up any mailbox as a catch all account. However, we HIGHLY recommend you delete the catch all account (or turn catch all off for all your mailboxes) because this mailbox could become a SPAM trap. Spammers send bulk mail to random accounts at a particular domain, so your catch all account would then trap all their SPAM and fill up your mailbox very quickly.
