You can grant or deny permissions for mailbox-enabled users. You
can then view permissions for a specific mailbox-enabled user.
Even if both the Allow and Deny check boxes are not
selected for a permission, the group or user may have obtained the
permission through group membership. If the group or user has not
inherited the permission, then the permission is implicitely
denied. If a check box is shaded, the permission is inherited and
must be changed at the level where the permission was set.