Easiest way to do this:
Start your message with <!--
Then display the message in plain text.
Close with -->
And begin your HTML version.
You may want to include a line of text at the end of the plain text
indicating that what follows is HTML.
The HTML comment tags will prevent it from being displayed by HTML-enabled
mail readers.
However, your non-HTML-enabled readers will display the message twice, once
as plain text in the HTML comment tags, and then the raw HTML code attached.
This annoys many such users. It would be preferable to allow the user to
store a preference for receiving HTML or plain text and only send that
version.