//SEIBERT/MEDIA has created a mail page add-on for Confluence which is also available on the Atlassian Marketplace.
Please bear in mind, that the idea of the plugin is to easily share content with others which includes externals. If you prevent the add-on to send mails to external people you limit its usecase dramatically. Also people can always export pages to PDF or Word and then send the content out through their mail client. So the risk of leaking content is not smaller by this type of prevention.
The feature is used as a replacement of internal newsletters that now can live natively in a Confluence Intranet like Linchpin.