The trick is in the header information. This script should get you started -- you'll need to create two copies of the email, one plain text and another in HTML format. This script will send one email containing both formats, and the client software will decide which to display based on the user's preferences. It saves you from having to ask if they want newsletters in plain text or HTML format.
$notice_text = "This is a multi-part message in MIME format."; $plain_text = "This is a plain text email.\r\nIt is very cool."; $html_text = "<html><body>This is an <b style='color:purple'>HTML</b> text email.\r\nIt is very cool.</body></html>";