Why is there a system email account for all domains I create?

For all domains created in DirectAdmin, you'll notice that there is always an email account with your DA username which cannot be deleted.  This is the system email account associated with your DA login.  The reason it's there is that it will always exist on the system as long as you have an account on the system.  The value shown as an email in DA is actually a forwarder for username@domain.com to username@server.hostname.com, where username is your DA login and the server.hostname.com is the hostname of your server.    All domains created under "username" will have this email all forwarded to the username@server.hostname.com email account.   For any email client, the login format is just "username", without any @domain.com or @server.hostname.com at the end.     It cannot be removed because this is where all emails for system tasks like cronjobs, cgi-bin scripts, suphp email replies, etc.. will end up.    Deleting it would cause bounces everywhere, and hide the fact that emails are piling up in the account, invisible to the User.

Note, that if you do not with to actually receive emails to username@domain.com, you can go to:
User Level -> Forwarders

and set:

username    ->   :fail:

where :fail: will reject all emails sent to it, as if it didn't exist.  You can alternatively use :blackhole: instead of :fail: to drop/ignore all emails without creating a bounce/rejection.

*** It's very important to note that creating a blocked fail/blackhole forwarder for your username@domain.com account does *not* delete the system account.  As mentioned, it cannot be deleted.   The username@server.hostname.com account will still exist, and will still get emails from cronjobs or any system account process running as "username".