Sure you can - public distribution lists are used for that. By default all subscribers on a Unity server are added to the "All Subscribers" distirbution list (unless you changed the subscriber templates after install). You can send a message to all users by addressing a voice mail to this group.
In 4.0(5) there's an additional "broadcast message" option which sends messages to all users on a server - it's a little different than sending a message to a public distribution list in that you can force the users to listen to the message (i.e. they can't skip through it) and the message is presented before they gain access to their inbox.
