Do you use the security options in Outlook?
When you distribute important information by email all you can do is put "Do Not Forward this please". Important corporate information should be protected better than this.
This solution exists in Microsoft Office and is built into Outlook. Entitled 'Information Rights Management', a file level security application built onto Windows Server. The capability enables you to prevent recipients of your emails (and attachments) from forwarding them on, copying any text, or printing the document (be aware that determined chaps could use a lower level screen shot program to get past this).
Additionally, it encrypts the file as it's sent away. As an added basis - you can secure on a group level (based on Active Directory groups). To prevent an email being forwarded simply create a new email and select the "options" tab and click on "permission" in the ribbon and select "do not forward".
Note: You may be interested to know that every mail item that you send gets a file saved with these credentials so you can still open the emails when you are offline. To see: go to Start - Run %USERPROFILE%\Local Settings\Application Data\Microsoft\drm.