blast! Also why do you want to send HTML email? A lot of people have that turned off and most of it doesn't render correctly in webapps. And it's a paaaain...
That said, if you really need HTML email... and it's absolutely needed...
Create a .html file and then Edit>Send Page via Email (in Safari). This will send it out in Mail.app.
If in Windows, Windows Live Mail has a "Edit Source" Mode.
Word of note :
1) if you have images, upload them online first. You can't link to images on your computer unless you pay for a static IP from your ISP and have Web Sharing turned on and leave your computer on all the time.
2) Must be built in tables. GMAIL and MobileME both strip out all CSS (though font-family and font-size are respected, and aren't inherited so every nested table needs these set).
3) Almost forgot, about the 3rd or 4th nested table the cellpadding is ignored. I have no idea why, but my solution was to add extra empty cells for padding.