Prevent Other Outlook Recipients from Forwarding, Printing, or Copying Content from Your Message?

"IRM is the feature in Outlook (since 2007) that has supported the features you describe. It relies on having access to an RMS server, typically a Windows Server machine (Rights Management Services have been around since Windows Server 2003).

In Outlook 2010, IRM operates through an Exchange server, although any RMS-supporting mail client should be able to receive the mails, as long as it has LAN/WAN/Internet access to the RMS server that manages the rights.

Here is an overview of exactly how it works. The examples are based on Outlook 2007, but the principle will be the same in 2010."