It's tricky in Outlook (Outlook Express lets you actually
edit the code directly if I recall correctly). But one clunky way
to do it regular Outlooks is to display your HTML page in your
browser. Not the source code, the resulting display page in a
normal browser window. Select all, copy, then paste into your HTML
format message in Outlook.
Keep in mind if you're sending this to more than 50 people or
so you stand a real good chance of getting that domain/IP black