prova a sostituire questa parte:

$body_top = "--Message-Boundary\n";
$body_top .= "Content-type: text/html; charset=iso-8859-1\n";
$body_top .= "Content-transfer-encoding: 7BIT\n";
$body_top .= "Content-description: Mail message body\n\n";


con:


$body_top = "MIME-Version: 1.0\r\n";
$body_top .= "Content-type: text/html; charset=iso-8859-1\r\n";
$body_top .= "Content-Transfer-Encoding: 7bit\r\n";