Thanks to my colleague Leiby Kandyba for pointing me to this nice feature for Outlook 2007 and 2010.:
The primary function is to add a couple of buttons to the Outlook ribbon to prevent people from doing a reply-all to your message, or forwarding it (using a facility built into Outlook & Exchange which is really lightweight compared to using IRM machinery, but which is not exposed in the existing UI). However, it also includes a check for email goofs such as omitting attachments or subject lines.
This works with both Outlook 2007 and Outlook 2010, as long as you’re using an Exchange account.
When you install this thing, you’ll see a couple of extra buttons at the end of the ribbon: No Reply All and No Forward. As the names suggest, clicking on these will prevent recipients of your emails from performing those two actions; clicking again toggles the relevant option off again.
Download it from here: unzip to somewhere on your local PC and run setup.exe.