The short option is that you are probably best off formatting the HTML messages and using the header parameter of the php mail function.
$headers = "From: sender@example.com \n" . "MIME-Version: 1.0\n" . "Content-type: text/html; charset=iso-8859-1"; mail( to@example.com , 'subject line', 'your message text <strong>with HTML in it</strong>', $headers);
The sitepoint.com site referenced by Jimmy provides an excellent and complete description of your capabilities.
source share