This is the only way that I know how:
1) Using Netscape Communicator begin to type your message.
2) Under the Tools menu select HTML Tools and then "Edit HTML Source..."
That will pull up a Java applet to edit the message. Type in your HTML there.
Outlook and them can do simple HTML mark-up stuff, like bold and italic type. But if you want to do tables and include images on your server and stuff this is the only way I've figured out to do it.