There are two options:
1. If you have Entourage 2004, you can create a new document in
Word, and then go to File > Send to > Mail Recipient (as HTML) to send the
document as HTML code in a message.
2. Paul Berkowitz has AppleScripts (E1.1 for 2001, X 2.0.1 for X or 2004)
that can insert HTML code from an HTML file into a message. Get the free
script here:
<http://www.scriptbuilders.net/category.php?search=Send+Complex+HTML>