When you're logged in as the system administrator, there IS a way to send a message to all users on the mail server. This is generally used to announce service schedules, etc. so that everyone on the server is kept in the loop when you need to take the server down for maintenance. You can find this feature by logging in as the system administrator, then going to:
Manage -> Mass Messaging -> Send Email
You can send messages to all users on all domains or even send to only users on specific domains, only domain administrators or even send messages to specific users.
I hope that helps.