The root user contains any error messages regarding your domain.  These messages are sent to "postmaster@domain.com" and are saved in the "root" folder.

By logging in to your email admin account, you have three options to manage this user's space:

1. Log in as admin, go to "User Administration," click on "root," go to "File Directory" and delete the files in the file directory. (Please note, this will need to be done regularly.)

OR 

2. Log in as admin, go to "User Administration," click on "root," go to "Alias Administration," and change "postmaster" to an account that you check regularly so that this new user will receive emails previously going to "postmaster" and landing in the "root" account. (Please note, this will increase the mail for the chosen user.)

OR

3. Log in as admin, go to "User Administration," click on "root," activate the account, create a password, and begin actually checking this email account. (Please note, this email account will need to be checked regularly.)