The final section of this page is for the mail-to-all mailmap, which sends a copy of any email sent to it to every destination address you have specified for that domain. To make use of this, you specify an alias in the "mail-to-all" text field, and then when ever you send email to that alias at your domain a copy of it will go to every mailmap you have setup in your account, including the default (if one is specified).