The SaaS Cloud service was not designed with this functionality in mind, so you're not likely to find a setting on the server that will work appropriately. Theoretically, if you have your system configured to move internal mail from mailbox to mailbox without ever going to the Internet, you can add the users to a group, and configure a Content Policy to deny every character, including a space, period, a-z, 0-9, etc. That may be the closest you can come to, but often it is better to configure users on the server level to restrict sending to the Internet.
You're asking for a routing feature, some call it a walled garden, that is most likely only configurable from your mail server or from a provider such as Google Apps. Such a feature has been widely used in EDU organizations.
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