A "catch-all" email address is where all mail goes that is sent to [email protected], where 'someone' doesn't exist. For example, suppose you have the following three email addresses set up:
Suppose further, that someone sends an email to the address:
That email would go to the 'default,' or 'catch-all' email address. In other words, any email sent to a non existent user/mailbox on your domain is sent to the catchall.
There is an option on your control panel under mail configuration to turn off the catch all address. If you do this, you will no longer receive any mail unless it is address specifically to an email account you have set up.