A catch-all account is created with each account by default. A catch-all account routes all mail not destined to a particular mailbox to your default mailbox (pop3). In this way you can receive mail to lots of addresses without having to setup corresponding users/mailboxes on the system. As a result of this, your default mailbox (pop3) could accumulate quite a bit of unwanted emails including Spam. If you wish to NOT receive email that is not targeted to a specific alias or pop3 account, you can change the catch-all to be forwarded to site_blackhole. The site_blackhole is just like the recycle bin in Microsoft Windows. This means that those emails will just be deleted.