While there always were tricky ways to send HTML email with the Mail.app even before Tiger, but Apple finally made it easy with 10.4. It only takes a minute if you follow these steps below:
- Compose you HTML email in the application of your preference.
- Include all styling in the HTML page.
- Upload it to your server (if you don’t use images you don’t need this step).
- Open the page in Safari.
- Press Apple(Cmd)-I.
- The web page will open in Mail.app as a new HTML message.
The title of the page will be the subject, but you can edit it as any other element of the page if you need to do any final touches. Make sure to send a test to yourself to sort out any unexpected results.