If you don't want to use the Mass Messaging you have two options:
- Create an email alias on the domain and select the option that reads "include all domain users". Any email sent to this alias will be automatically forwarded to all email accounts on the domain.
- If you don't want others being able to send emails to all domain users then you would want to setup a Mailing List and set yourself as the Moderator and limit posters to Moderator Only. The downside of this is that you would have to export a list of your users and import them into your Mailing List Subscribers.