DocTypes don't really work in html emails, nor does anything in the head tag really.
A good html email should look like this as it's base
<html>
<head>
<title>Title of email</title>
</head>
<body>
</body>
</html>
Then, for the design/layout. You can't use any styles for the most part. Only font-family, font-size, line-height, color. For spacing, use BR tags. <p> Tags aren't rendered in Yahoo email clients and margin/padding isn't rendered in the majority of the others. You literally have to use height attributes or BR tags for spacing.
Only use tables, and old old html attributes (like ones that are almost all depreciated. If you want some space ontop of an image for example, use the
vspace attribute.
I probably have more experience than anyone else here with html email design and layouts. People that think designing for a couple browsers is though, when I run an email design it has to be tested and functioning in 30+ different email clients. All of which have insane quirks compared to browsers.
Oh, add style="display:border;" to all images that are part of the layout, other wise you'll end up with spacing around them in about 5 of the clients.
Oh, and always provide a expandable area for the text/content area. MobileMe and Lotus Notes suck for spacing and they'll break a fixed size content region.