Use this script to send HTML emails using PHP. An email will be sent to the address specified every time the page is loaded.
<?php

$from = "Your e-mail";
$subject = "Subject";
$body = 'your html code goes in here';

$headers="Content-Type:text/html;CHARSET=iso-8859-1\r\n";
$headers.="From:".$from."\r\n";

mail($email, $subject, $body, $headers);

?>

If you wanted to make it a newsletter script you would could insert all your emails into a mysql table, then execute the above code for each email using the while() statement.