By default, the root user (System Administrator) is not mail enabled and that is good.
You should never enable mail reception for root directly.
This is best accomplished by adding an alias which will forward mail to another user.
To do so, edit
save and issue:
sudo postalias aliases
sudo postfix reload
If you look inside
, you will notice that all common system accounts (postmaster, hostmaster, etc) are aliased to root. So by making these changes you will receive mails for those users too. You can of course change the aliases in any way you like.