Prueba esto...
Código PHP:
$headers = array ('From' => $from,
'To' => $to,
'Subject' => $subject);
$smtp = Mail::factory('smtp',
array ('host' => $host,
'auth' => true,
'username' => $username,
'password' => $password));
$crlf = "\n";
$mime = new Mail_mime($crlf);
$mime->setHTMLBody($body);
$body = $mime->get();
$headers = $mime->headers($headers);
$mail = $smtp->send($to, $headers, $body);