Mine

$boundary = md5(microtime());

$headers .= "From: Biotech Association of Maine<noreply@noreply.com>" . "\r\n";
$headers .= "To: " . $_REQUEST['Email:'] . "\r\n";
$headers .= "MIME-Version: 1.0 \r\n";
$headers .= "Content-Type: multipart/alternative; boundary=\"" . $boundary . "\" \r\n";

$msg .= "--$boundary\r\n";
$msg .= "Content-Type: text/plain; charset=iso-8859-1\r\n";
$msg .= "Content-Transfer-Encoding: 8bit\r\n";
$msg .= $body . "\r\n";
$msg .= "--$boundary\r\n";
$msg .= "Content-Type: text/html; charset=iso-8859-1 \r\n";
$msg .= "Content-Transfer-Encoding: 8bit\r\n";
$msg .= $htmlbody . "\r\n";

$msg .= "--$boundary--\r\n";