An alias is a ‘virtual’ mail box, which serves only to forward messages on to the alias target. By default, there is a ‘postmaster’ alias which references the ‘root’ mail box. So both ‘postmaster@<domain-name>’ and ‘root@<domain-name>’ will be stored in the ‘root’ mail box.
A useful feature of aliases is the ‘nobody’ functionality. Setting up an alias of ‘nobody’ will collect all messages for the domain which don’t match another alias or mail-box name and send them to the target e-mail address or box.
Clicking the Alias Administration link in mail server interface will allow you to create mail aliases.