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 firstname.lastname@example.org to email@example.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 firstname.lastname@example.org 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.
It is recommended that when you create a main email address that you foward emails from the system email address to your newly created email address so that you still recieve system notices.