The oldest and most popular is simple text. Many people still use older email software that can only handle plain text. It chokes on different font sizes or colours. And don't even think about including images. If your readers are using a mix of older and more recently developed email applications, then text based email is a safe choice.
Text based email is also easy and inexpensive for new cyber-marketeers to cut their teeth on.
There are only few simple conventions to keep in mind. Keep text to a minimum and use narrow margins of about 60 characters to increase readability. Separate paragraphs with extra spacing or simple design characters like asterisks and underlining. Offer a sexy, interesting bit of copy followed by a hypertext link to entice readers to click and visit your Web site for more details. Driving readers to your Web site is the goal because this is where your information is well presented and branded.