That's because the read-only status corresponds to the state of the file on your hard drive. When you attach it to a mail message, you're sending a copy. Use Tools | Protect Document on the document itself instead.
--
Sue Mosher, Outlook MVP
Outlook and Exchange solutions at http://www.slipstick.com
Author of
Microsoft Outlook Programming: Jumpstart
for Administrators, Power Users, and Developers http://www.slipstick.com/books/jumpstart.htm