Go to the "Manage" page. Click on "E-mail Gateway Settings". There's a link at the top of the page that's supposed to go to instructions on how to use the feature. It just leads back to the main "Magage" page again.
I got myself caught in a loop for a while, until I went back to the "News" post regarding the feature and followed the instructions link there.
I'm sorry if this is the wrong community - I'm still not sure how or when to use 'Zilla... or if I should have opened a Support request.. or just left it alone...