Paul O'B is heading in the right direction here. The easiest way to create and send an email newsletter is to a) build it as an HTML file, then b) use an app like Campaign Monitor or Mailchimp to test, then finally send the email campaign. My gang (Campaign Monitor) don't charge until you send to more than 5 email addresses in one hit, so effectively, you could load up your HTML file there and test to your heart's content.
The problem with just loading your code into a desktop client like Thunderbird is that it can be pretty limiting, while apps like CM do tasks that you probably haven't considered yet. Some examples are copying CSS inline for compatibility, or creating the text-version (Multipart-MIME, as Michael Morris suggested).
You can by all means use CSS, however check this CSS in email guide to find out which properties work across the major email clients.
For a good primer on HTML email coding, [check out this post by 24ways. Alternately, you can [URL="http://www.campaignmonitor.com/templates/"]download a free template](http://24ways.org/2009/rock-solid-html-emails) and start hacking away from there.
Again, I'm from Campaign Monitor, the community gal to be exact. We're all designers who have spent time cutting our teeth on email, so pop us a line if you have any questions.