After that and rebooting your outlook and clicking the icon again , you can receive the message error:
"Unexpected error occurred. Please try again later or contact your system administrator"
In order to avoid that you should install the below update
Description of the Office 2003 documents protected with AD RMS/RMS update package
After restart your outlook you will able to restrict your message, bear in mind when the user received this message he/she will not FW, print neither copy the message body..Interesting isn't it..
I'll post the transport rules in order to force this restriction to some users or groups in AD
Further info firstname.lastname@example.org