When a domain is created a default set of mailboxes are created as follows:
Admin (or the name specified when defining the root.login name in the configuration wizard)
This is the default administrator for the domain
Administrators
This is a group mailbox containing all the administrators of the domain. Members of this group are only granted access to WebAdmin if the mailbox itself is granted administrator access. See Web Administration
dsn
A special mailbox for handling list mailbox delivery status notifications
everyone
A group mailbox into which FTGate adds all new user mailboxes created. Mailboxes can be removed from the group to hide them from other users.
null
A mailbox that deletes all mail sent to it.
postmaster
An alias for administrators. Internet RFC's require that a postmaster be defined.
system
A mailbox that processes system commands. See Mailbox Overview