What kind of restriction are you looking for? Emails are external to Adobe (unless of course you work for them).
I am unaware of any restrictions on what emails can be added to a report.. and besides, once it's sent, there is nothing to stop someone from forwarding the email elsewhere or copying the file to a new email and sending it off....
I think about the most you can do is restrict Data Warehouse access to trusted users, and make sure that they are careful about who they send data too... but again, once it's out there....
I am not sure if you can modify the email template for your organization to contain a boilerplate about confidentiality... its not really a restriction.. but if someone does forward it, they are technically responsible for passing on confidential information..... You could check with client care on whether or not templates can be modified....
If you can't modify them, then if you are really concerned about driving home the "confidential" factor, I suppose you could set up a process by which all warehouse exports come to trusted team members, then they attach the file into a company branded email with the confidential disclaimer on it....
I know this isn't really what you are looking for.. but I don't think such a restriction currently exists.....
